Add support for GObjectIntrospection

Turned all GnomeRR structures into boxed types, then added the needed annotations and the Makefile.am bits.
Does not yet include API changes, but should bind (awfully) all of libgnome-desktop.

Based on gnome commit: cad94246fb5be76482212407a380cd75f9e7b932
url: https://git.gnome.org/browse/gnome-desktop/commit/?id=cad94246fb5be76482212407a380cd75f9e7b932