actor: Clarify and improve memory management of transitions
actor: Clarify and improve memory management of transitions

When adding a transition to a ClutterActor, the actor should hold a
reference on it, and release it only when we remove it. This makes
transitions just like other objects held by ClutterActor.

djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded