[Overview] Make content area eat mouse events during animations

[Overview] Make content area eat mouse events during animations

There are ton of different kinds of mouse even handlers in the overview;
WindowClone has several mouse-enter/leave handlers, we still have a variety
of classes not ported to ButtonBox and so incorrectly handling double-click,
etc.

Since we at present don't have anything in the overview area for which
it makes sense to interact with during the animation, create a transparent
event-eating box which we raise to the top during the animation.

https://bugzilla.gnome.org/show_bug.cgi?id=594074