ShellIdleMonitor: turn it into a singleton

It doesn't make sense to have multiple ShellIdleMonitors, since
each has its own GDK filter function, but they all get the same
events. In preparation for having it accessed from other places
than the message tray, make it a singleton.

https://bugzilla.gnome.org/show_bug.cgi?id=682041

js/ui/messageTray.js

@@ -1508,7 +1508,7 @@ const MessageTray = new Lang.Class({
         this._idleMonitorWatchId = 0;
         this._backFromAway = false;
 
-        this.idleMonitor = new Shell.IdleMonitor();
+        this.idleMonitor = Shell.IdleMonitor.get();
 
         // To simplify the summary item animation code, we pretend
         // that there's an invisible SummaryItem to the left of the