The Constraint should plug directly into the allocation mechanism, and
modify the allocation of the actor to which they are applied to. This is
similar to the mechanism used by the Effect class to modify the paint
sequence of an actor.
The ClutterActor API should have modifier methods for adding, removing
and retrieving Actions and Constraints using the ClutterActorMeta:name
property - mostly, for convenience.
The Constraint base, abstract class should be used to implement Actor
modifiers that affect the way an actor is sized or positioned inside a
fixed layout manager.
