Use clutter_actor_destroy in dispose, add _dispose where needed
ClutterGroup calls _destroy, but most of St was just calling _unparent. This caused problems because the DESTROY signal was not emitted for child elements after destroying a toplevel. Also, in a GC'd binding it would cause unpredictable lifetime of children. Some St widgets simply didn't have _dispose at all; implement it. Note because of the usage of the background_image in StButton, we can't cleanly destroy it inside the StWidget. https://bugzilla.gnome.org/show_bug.cgi?id=597845
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