clutter/actor: Disconnect from "layout-changed" signal on destroy

While the layout manager of a ClutterActor does get properly unset when destroying an actor, we currently forget to disconnect the "layout-changed" signal from it.

So do that, and while at it, also switch to using the signal id for disconnecting from the signal instead of g_signal_handlers_disconnect_by_func(), which caused problems before because it might traverse the signal handler list.