Reorganize overlay hiding/showing code

Rather than having main.js manage this, put it into overlay.js, and
have the overlay object emit signals that other code can watch to do
things when the overlay is showing/shown/hiding/hidden.

js/ui/windowManager.js

@@ -86,7 +86,7 @@ WindowManager.prototype = {
     },
 
     _shouldAnimate : function(actor) {
-        if (Main.overlayActive)
+        if (Main.overlay.visible)
             return false;
         if (actor && (actor.get_window_type() != Meta.CompWindowType.NORMAL))
             return false;