Made the device list self-updating

Previously, the device list only loaded once on startup.  This was poor
behaviour, as devices can be attached and detached at any time.

Now the attached devices are polled once per second, and the list is
updated accordingly.  The code behind it is a bit messy because the
list model interacts directly with parts of the GUI, but it works
pretty well nevertheless.  There's no mechanism to kick us out of the
configuration screen if the device we're configuring is detached, but
this can be addressed later.  For now, I'm reasonably happy with this
new feature.

pd-buddy-gtk.py

 import serial.tools.list_ports
 import gi
 gi.require_version('Gtk', '3.0')
-from gi.repository import Gtk
+from gi.repository import Gtk, Gio, GObject, GLib
 
 
 def update_header_func(row, before, data):
     return answer
 
 
+class ListRowModel(GObject.GObject):
+
+    def __init__(self, serport):
+        GObject.GObject.__init__(self)
+        self.serport = serport
+
+
+class SelectListStore(Gio.ListStore):
+
+    def __init__(self, stack, list_empty, list_frame):
+        Gio.ListStore.__init__(self)
+
+        self.stack = stack
+        self.list_empty = list_empty
+        self.list_frame = list_frame
+
+        self.update_items()
+        GLib.timeout_add(1000, self.update_items)
+
+    def update_items(self):
+        # Clear the old entries
+        self.remove_all()
+
+        # Search for the serial ports
+        for serport in serial.tools.list_ports.grep("1209:0001"):
+            self.append(ListRowModel(serport))
+
+        # Set the visible child
+        # FIXME: This is rather poor organization
+        if self.get_n_items():
+            self.stack.set_visible_child(self.list_frame)
+        else:
+            self.stack.set_visible_child(self.list_empty)
+
+        return True
+
+
 class SelectListRow(Gtk.ListBoxRow):
 
-    def __init__(self, serial_port):
+    def __init__(self, lrm):
         Gtk.EventBox.__init__(self)
 
-        self.serial_port = serial_port
+        self.serial_port = lrm.serport
 
         self._builder = Gtk.Builder()
         self._builder.add_from_file("data/select-list-row.ui")
         self._builder.connect_signals(self)
 
         name = self._builder.get_object("name")
-        name.set_text(serial_port.description)
+        name.set_text(self.serial_port.description)
 
         device = self._builder.get_object("device")
-        device.set_text(serial_port.device)
+        device.set_text(self.serial_port.device)
 
         self.add(self._builder.get_object("grid"))
         self.show_all()
         # Get the list
         sl = self.builder.get_object("select-list")
         ss = self.builder.get_object("select-stack")
+        se = self.builder.get_object("select-none")
         sf = self.builder.get_object("select-frame")
 
-        # Search for the serial ports
-        for serport in serial.tools.list_ports.grep("1209:0001"):
-            # Show the list if we have a serial port
-            ss.set_visible_child(sf)
-            sl.insert(SelectListRow(serport), -1)
+        liststore = SelectListStore(ss, se, sf)
+
+        sl.bind_model(liststore, SelectListRow)
 
         # Add separators to the list
         sl.set_header_func(update_header_func, None)
         st.set_visible_child(select)
 
     def on_header_sink_save_clicked(self, button):
-        combo = self.builder.get_object("voltage-combobox")
-        spin = self.builder.get_object("current-spinbutton")
-        print("{} V".format(combo.get_active_text()))
-        print("{} A".format(spin.get_value()))
         rev = self.builder.get_object("header-sink-save-revealer")
         rev.set_reveal_child(False)