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.