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
ab0460ad59