Don't update Save button until command sent

Previously, we updated the Save button before sending the commands.
This meant that if the command failed, the save button was still updated
before the error dialog appeared, which was a bit odd.  Now that's
fixed.

pd-buddy-gtk.py

         st.set_visible_child(select)
 
     def on_header_sink_save_clicked(self, button):
-        self._store_device_settings()
-        self._set_save_button_visibility()
-
         window = self.builder.get_object("pdb-window")
         try:
             pdb_send_message(window, self.serial_port, 'write')
+
+            self._store_device_settings()
+            self._set_save_button_visibility()
         except:
             self.on_header_sink_back_clicked(None)
 
                              or current.get_value() != self.current)
 
     def on_voltage_combobox_changed(self, combo):
-        self._set_save_button_visibility()
-
         window = self.builder.get_object("pdb-window")
         try:
             pdb_send_message(window, self.serial_port,
                              'set_v {}'.format(int(combo.get_active_text())*1000))
+
+            self._set_save_button_visibility()
         except:
             self.on_header_sink_back_clicked(None)
 
     def on_current_spinbutton_changed(self, spin):
-        self._set_save_button_visibility()
-
         window = self.builder.get_object("pdb-window")
         try:
             pdb_send_message(window, self.serial_port,
                              'set_i {}'.format(int(spin.get_value()*1000)))
+
+            self._set_save_button_visibility()
         except:
             self.on_header_sink_back_clicked(None)