Use GObject Introspection

Without GObject Introspection, newer GObject-based Gtk3 apps will not work (e.g. Totem) as it is impossible to use both static pygtk and GObjects Introspection Gtk/Gdk.

This change makes Totem coherence_upnp plugin working again (even though they have removed the plugin due to coherence being broken for a long time).  Only TreeWidget was tested.

Also it was required to re-fetch GtkTreePath since its internal structures became lost after dbus call returned. This may be a bug in python overrides for gir though, but with the change introduced it is working fine.