Replace GnomeIdleMonitor with MetaIdleMonitor

Now that GnomeIdleMonitor is a DBus API for mutter, we need to
use own in-process thing, to avoid dead locks.

https://bugzilla.gnome.org/show_bug.cgi?id=706005

js/ui/messageTray.js

@@ -1685,7 +1685,7 @@ const MessageTray = new Lang.Class({
 
         this._userActiveWhileNotificationShown = false;
 
-        this.idleMonitor = new GnomeDesktop.IdleMonitor();
+        this.idleMonitor = Meta.IdleMonitor.get_core();
 
         this._grabHelper = new GrabHelper.GrabHelper(this.actor,
                                                      { keybindingMode: Shell.KeyBindingMode.MESSAGE_TRAY });