Bug 591437 - Rename overlay.js to overview.js

Replace 'overlay' with the more descriptive name 'overview'
where the Activities Overview is meant. Call it Overview
(capitalized) in code comments.

The overlay-group and overlay-key provided by Mutter are not
affected, since they may be used for other components than
the Activities Overview.

js/ui/windowManager.js

@@ -85,7 +85,7 @@ WindowManager.prototype = {
     },
 
     _shouldAnimate : function(actor) {
-        if (Main.overlay.visible)
+        if (Main.overview.visible)
             return false;
         if (actor && (actor.get_window_type() != Meta.CompWindowType.NORMAL))
             return false;