When destroying an Actor the various ActorMeta instance should already
be disposed - unless something is holding a reference to them, in which
case we should use the ::destroy signal to unset the ActorMeta:actor
back pointer.
ClutterActorMeta is a base, abstract class that can be used to derive
classes that are attached to a ClutterActor instance in order to modify
the way an actor is painted, sized/positioned or responds to events.
A typed container for ActorMeta instances is also provided to the
sub-classes can be attached to an Actor.
