glib: install interfaces earlier, and avoid GetClassPtr() at init time

GObject upstream has started disabling support for installing interfaces
in GTypes after they have already been initialized (class_init) [1], so
we need to add GInterfaces a bit earlier during initialization, and avoid
the use of GetClassPtr() (whose calls to either g_type_class_peek or
g_type_class_ref causes the class_init to be called).

[1] http://bugzilla.gnome.org/687659

Simplest way to test this is launching the sample/treemodeldemo.exe in
Ubuntu 13.04 beta. Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=11510