Refactor the overview window overlays to use ClutterConstraints

Refactor the window overlays of the overview to use Clutter constraints to position the different parts of the overlay.

Implement this by making the WindowOverlay class a subclass of ClutterActor and setting (or animating) its position and size just like we set it for the WindowClones (we can't use a BindConstraint to bind the overlays directly to the clones because clones are scaled and the overlay gets an allocation equal to the position and size of the scaled clone).

Then add a container with the same size as the WindowOverlay actor as a child to the actor and use this relativeContainer as the source for the border actor constraints (the container has the position of 0,0, which we need if we want to add constraints to the children). The close button and window title actors are positioned using constraints bound to the border actor.

This also allows us to remove the slot property of WindowClone.

Depends on (and includes the commits of) !683 (merged)