Added GiveBack flag GtkToggleButton

Since the firmware supports GiveBack now, the GUI should as well.  This
commit adds a toggle button for toggling the GiveBack flag on a PD Buddy
Sink.

data/pd-buddy-gtk.ui

       <object class="GtkStack" id="stack">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
-        <property name="transition_type">slide-left-right</property>
+        <property name="transition_type">crossfade</property>
         <child>
           <object class="GtkScrolledWindow" id="select">
             <property name="visible">True</property>
                                   <object class="GtkLabel">
                                     <property name="visible">True</property>
                                     <property name="can_focus">False</property>
+                                    <property name="halign">end</property>
                                     <property name="label" translatable="yes">Voltage</property>
                                     <property name="justify">right</property>
                                     <style>
                                   <object class="GtkLabel">
                                     <property name="visible">True</property>
                                     <property name="can_focus">False</property>
+                                    <property name="halign">end</property>
                                     <property name="label" translatable="yes">Current</property>
                                     <property name="justify">right</property>
                                     <style>
                                     <property name="top_attach">0</property>
                                   </packing>
                                 </child>
+                                <child>
+                                  <object class="GtkLabel">
+                                    <property name="visible">True</property>
+                                    <property name="can_focus">False</property>
+                                    <property name="halign">end</property>
+                                    <property name="label" translatable="yes">Flags</property>
+                                    <property name="justify">right</property>
+                                    <style>
+                                      <class name="dim-label"/>
+                                    </style>
+                                  </object>
+                                  <packing>
+                                    <property name="left_attach">0</property>
+                                    <property name="top_attach">2</property>
+                                  </packing>
+                                </child>
+                                <child>
+                                  <object class="GtkButtonBox">
+                                    <property name="visible">True</property>
+                                    <property name="can_focus">False</property>
+                                    <property name="halign">start</property>
+                                    <property name="orientation">vertical</property>
+                                    <property name="layout_style">end</property>
+                                    <child>
+                                      <object class="GtkToggleButton" id="giveback-toggle">
+                                        <property name="label" translatable="yes">GiveBack</property>
+                                        <property name="visible">True</property>
+                                        <property name="can_focus">True</property>
+                                        <property name="receives_default">True</property>
+                                        <signal name="toggled" handler="on_giveback_toggle_toggled" swapped="no"/>
+                                      </object>
+                                      <packing>
+                                        <property name="expand">True</property>
+                                        <property name="fill">True</property>
+                                        <property name="position">0</property>
+                                      </packing>
+                                    </child>
+                                  </object>
+                                  <packing>
+                                    <property name="left_attach">1</property>
+                                    <property name="top_attach">2</property>
+                                    <property name="width">2</property>
+                                  </packing>
+                                </child>
                               </object>
                             </child>
                           </object>
       <object class="GtkStack" id="header-stack">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
+        <property name="transition_type">crossfade</property>
         <child>
           <object class="GtkHeaderBar" id="header-select">
             <property name="visible">True</property>

pd-buddy-gtk.py

         self.serial_port = None
         self.voltage = None
         self.current = None
+        self.giveback = None
         self.selectlist = None
 
     def on_pdb_window_realize(self, *args):
         # Get voltage and current widgets
         voltage = self.builder.get_object("voltage-combobox")
         current = self.builder.get_object("current-spinbutton")
+        giveback = self.builder.get_object("giveback-toggle")
 
         self.serial_port = serport
 
 
         # Get voltage and current from device and load them into the GUI
         for line in tmpcfg:
-            if line.startswith(b'v:'):
+            if line.startswith(b'flags:'):
+                line = line.split()[1:]
+                try:
+                    line.index(b'GiveBack')
+                    giveback.set_active(True)
+                except:
+                    giveback.set_active(False)
+            elif line.startswith(b'v:'):
                 v = line.split()[1]
                 if v == b'5.00':
                     voltage.set_active_id('voltage-five')
         # Get voltage and current widgets
         voltage = self.builder.get_object("voltage-combobox")
         current = self.builder.get_object("current-spinbutton")
+        giveback = self.builder.get_object("giveback-toggle")
 
         # Remember the loaded settings
         self.voltage = voltage.get_active_id()
         self.current = current.get_value()
+        self.giveback = giveback.get_active()
 
     def _set_save_button_visibility(self):
         """Show the save button if there are new settings to save"""
         # Get relevant widgets
         voltage = self.builder.get_object("voltage-combobox")
         current = self.builder.get_object("current-spinbutton")
+        giveback = self.builder.get_object("giveback-toggle")
         rev = self.builder.get_object("header-sink-save-revealer")
 
         # Set visibility
         rev.set_reveal_child(voltage.get_active_id() != self.voltage
-                             or current.get_value() != self.current)
+                             or current.get_value() != self.current
+                             or giveback.get_active() != self.giveback)
 
     def on_voltage_combobox_changed(self, combo):
         window = self.builder.get_object("pdb-window")
         except:
             self.on_header_sink_back_clicked(None)
 
+    def on_giveback_toggle_toggled(self, toggle):
+        window = self.builder.get_object("pdb-window")
+        try:
+            pdb_send_message(self.serial_port, 'toggle_giveback', window)
+
+            self._set_save_button_visibility()
+        except:
+            self.on_header_sink_back_clicked(None)
+
 class Application(Gtk.Application):
 
     def __init__(self, *args, **kwargs):