Main.activateWindow: always exit the overview if it's currently open

This way, clicking a message tray icon while the overview is open will
close the overview when activating its window.

Remove some other overview-related activation code which is now
redundant.

Also, remove calls to "global.get_current_time()" when calling
Main.activateWindow, since it's unnecessary (activateWindow will call
it itself if you don't pass in that arg).

https://bugzilla.gnome.org/show_bug.cgi?id=609765

js/ui/workspace.js

@@ -1419,7 +1419,7 @@ Workspace.prototype = {
     },
 
     _onCloneSelected : function (clone, time) {
-        Main.overview.activateWindow(clone.metaWindow, time);
+        Main.activateWindow(clone.metaWindow, time);
     },
 
     _removeSelf : function(actor, event) {