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.
Since constructing AlignConstraint and BindConstraint instances could be
deferred (think ClutterScript) we need to make their :source properties
setters accept NULL. This does not break the constraints because they
need to handle that condition in case they actor to which they are
applied is destroyed and somebody is holding a reference on them anyway.
