Instead of using signal notifications, we should be able to keep track
of the clones of an actor from within ClutterActor itself, using private
API. There's no point in pretending that people can actually create a
Clone class out of tree, given the amount of invariants we have to punch
through in order to implement a proper replicator node of the scene
graph, so we can just skip the signal emissions and just do the right
thing at the right time.