ctrlAltTab: don't allow more than one popup

In a normal user session you can't have more than one
popup, because the popup is modal and we don't allow
the popup to show up when there are other modals.

In a GDM session, however, the login dialog is modal, and
we want a popup, so we don't have that same check.

This commit changes the ctrlAltTab manager code to not
allow multiple popups.

https://bugzilla.gnome.org/show_bug.cgi?id=659177

js/ui/ctrlAltTab.js

@@ -123,7 +123,16 @@ CtrlAltTabManager.prototype = {
             return;
 
         items.sort(Lang.bind(this, this._sortItems));
-        new CtrlAltTabPopup().show(items, backwards);
+
+        if (!this._popup) {
+            this._popup = new CtrlAltTabPopup();
+            this._popup.show(items, backwards);
+
+            this._popup.actor.connect('destroy',
+                                      Lang.bind(this, function() {
+                                          this._popup = null;
+                                      }));
+        }
     }
 };