animation: Allow detaching an animation from an actor

animation: Allow detaching an animation from an actor

When animating an actor through clutter_actor_animate() and friends we
might want forcibly detach the animation instance from the actor in
order to start a new one - for instance, in response to user
interaction.

Currently, there is no way to do that except in a very convoluted way,
by emitting the ::completed signal and adding a special case in the
signal handlers; this is due to the fact that clutter_actor_animate()
adds more logic than the one added by clutter_animation_set_object(),
so calling set_object(NULL) or unreferencing the animation instance
itself won't be enough.

The right way to approach this is to add a new method to Clutter.Actor
that detaches any eventual Animation currently referencing it.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2051