Constraints can only update an existing allocation, which means they
live only halfway through the layout management system used by Clutter;
this limitation makes it impossible, for instance, to query the
preferred size of an actor, if the actor is only using constraints to
manage its own size.
And move the only private ClutterConstraint method to it.
This commit also sneaks in a change that makes sense for the debugging
of the update_allocation() method, which checks if the allocation was
effectively changed.
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.
