ClutterActor took a reference in its transition 'stopped' handler,
aiming to keep the transition alive during signal emission even if it
was removed during. This is, however, already taken care of by
ClutterTimeline, by always taking a reference during its 'stopped'
signal emission, so no need to add another one.

This also has the bonus of making reference ownership simpler, as well
as avoidance of double free if an actor was destroyed before a
transition has finished.