alternateTab: Fix Escape not dismissing popup

Since shell commit dd85670f8b25, the handler got a return value to
determine whether Escape should be handled automatically; we do
want this for the window switcher, so add an appropriate return
value to restore the expected behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=737457

extensions/alternate-tab/extension.js

@@ -44,7 +44,11 @@ function enable() {
                 this._select(this._previous());
             else if (keysym == Clutter.Right)
                 this._select(this._next());
+            else
+                return Clutter.EVENT_PROPAGATE;
         }
+
+        return Clutter.EVENT_STOP;
     };
 
     setKeybinding('switch-applications', Lang.bind(Main.wm, Main.wm._startWindowSwitcher));