MetaIdleMonitor: protect the monitor instance while calling destroy notify

The destroy notify for a DBus watch holds a reference to the IdleMonitor,
but the IdleMonitorWatch object doesn't (it knows all watches will
be destroyed before the monitor is, so it doesn't need one). This
means that the DBus watch reference can be the only one keeping
the IdleMonitor alive (expecially true for device idle monitors,
which are only used by g-s-d/cursor), and that means that calling
the destroy notify freezes the monitor (and the next X calls
access garbage).

https://bugzilla.gnome.org/show_bug.cgi?id=708420