altTab: popModal before fading out

This lets the user start typing in the newly-selected window right
away, without any characters possibly getting eaten during the
animation.

https://bugzilla.gnome.org/show_bug.cgi?id=644857

js/ui/ctrlAltTab.js

@@ -281,6 +281,7 @@ CtrlAltTabPopup.prototype = {
     },
 
     _onDestroy : function() {
+        this._popModal();
         if (this._keyPressEventId)
             this.actor.disconnect(this._keyPressEventId);
         if (this._keyReleaseEventId)