Instead of using the fixed position and size API, use the newly added
update_allocation() virtual function in ClutterConstraint to change the
allocation of a ClutterActor. This allows using constraints inside
layout managers, and also allows Constraints to react to changes in the
size of an actor without causing relayout cycles.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2319
Allow using the BindConstraint to bind width and height of a source
actor.
Also, add a test for the BindConstraint showing all types of usages
for this constraint class.
