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.
AlignConstraint is a simple constraint that keeps an actor's position
aligned to the width or height of another actor, multiplied by an
alignment factor.
