[cherry-pick] st-bin: Destroy child in ClutterActor:destroy vfunc

According to Clutter documentation, "[…] actors implementing the ClutterContainer interface should override the default implementation of the class handler of this signal and call clutter_actor_destroy() on their children."

StBin was doing that in GObject:dispose() instead. Move the child destruction to a new ClutterActor:destroy() vfunc override.

(cherry picked from commit b719744e)