Add a basic Presence Service tab to the developer console.

* Newly added activities and buddies appear in bold for 5 seconds.
* Removed activities and buddies stay in the list with strikethrough applied
  for 5 seconds, after which they vanish.
* Columns other than "object path" don't do anything.
* In the object path column, the common prefix is replaced by ".../".

services/console/README

@@ -0,0 +1,12 @@
+Defining new tabs in the developer console
+==========================================
+
+The tabs are top-level packages inside 'interface/'.
+
+Each package used as a tab must have a class Interface, instantiatable
+with no arguments, with an attribute 'widget' that is a Gtk widget to be
+placed in the tab. That's it.
+
+Tabs are automatically run under the GLib main loop, dbus-python is set up
+to use it, and the shared dbus-python session-bus connection is expected to
+exist.