Don't completely refresh the list every second

Previously, we refreshed the list by completely erasing it and building
a new one.  While simple, it caused some obnoxious visual glitches, and
I could imagine some more serious problems it might have caused as well.

Now we remove only the devices that don't exist any more, and add only
those that we don't already know about.  Much better for the user
interface, but O(n^2).  Oh well, I doubt anyone will have enough PD
Buddies connected at once for that to matter much.

pd-buddy-gtk.py

@@ -57,12 +57,29 @@ class SelectListStore(Gio.ListStore):
         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))
+        # Get a list of serial ports
+        serports = list(serial.tools.list_ports.grep("1209:0001"))
+
+        # Mark ports to remove or add
+        remove_list = []
+        list_len = self.get_n_items()
+        for i in range(list_len):
+            remove = True
+            for j in range(len(serports)):
+                if serports[j] is not None and self.get_item(i).serport == serports[j]:
+                    serports[j] = None
+                    remove = False
+            if remove:
+                remove_list.append(i)
+
+        # Remove the missing ones
+        for i in remove_list:
+            self.remove(i)
+
+        # Add any new ports
+        for port in serports:
+            if port is not None:
+                self.append(ListRowModel(port))
 
         # Set the visible child
         # FIXME: This is rather poor organization