We need to make sure that ClutterActor::transition-stopped is emitted
after the transition was removed from the actor's list of transitions.
We cannot just remove the TransitionClosure from the hash table that
holds the transitions associated to an actor, and let the
TransitionClosure free function stop the timeline, thus emitting the
::transition-stopped signal - otherwise, stopping the timeline will end
up trying to remove the transition from the hash table, and we'll get
either a warning or a segfault.
Since we know we're removing the timeline anyway, we can emit the signal
ourselves in case the timeline is playing, in both the implicit and
explicit cases.
