st-bin: Destroy child in ClutterActor:destroy vfunc

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.