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 example code that is meant to be XIncluded into the API reference
should not be part of the interactive test suite: it's code that it is
meant to be used as a reference implementation - whereas the interactive
test suite should be allowed to be lean and test behaviour even in nasty
ways. In short: the test suite should not be the place where we show off
idiomatic code for educational purposes.
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 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.
