Now that we reinstated Group to its "former glory", we need to ensure
that applications using the deprecated containers with the new DOM API
in ClutterActor can actually work - or, at least, not break horribly.
This actually means making sure that ClutterStage and ClutterGroup can
cope with the DOM, while retaining their old implementations, as well as
their bizarre idiosyncrasies and their utter, utter brokenness.
Just like ClutterBindConstraint has two shorthand enumerations for binding
position and size and avoid using multiple instances, ClutterAlignConstraint
should have a way to align an actor with the same alignment factor on both
axis at the same time; this is especially useful for centering actors.
