messageTray: fix notification/keyboard interaction

If the pointer moves from the notification into the keyboard, don't
treat that as moving out of the tray.

https://bugzilla.gnome.org/show_bug.cgi?id=658603

js/ui/layout.js

@@ -45,7 +45,9 @@ LayoutManager.prototype = {
         this.trayBox.connect('allocation-changed',
                              Lang.bind(this, this._updateTrayBarrier));
 
-        this.keyboardBox = new St.BoxLayout({ name: 'keyboardBox' });
+        this.keyboardBox = new St.BoxLayout({ name: 'keyboardBox',
+                                              reactive: true,
+                                              track_hover: true });
         this.addChrome(this.keyboardBox, { visibleInFullscreen: true });
         this._keyboardHeightNotifyId = 0;