Add a new helper for tracking user idle activity

When running as a wayland compositor, we can't use the xserver's
IDLETIME, because that's updated only in response to X events.
But we have all the events ourselves, so we can just run the timer
in process.

https://bugzilla.gnome.org/show_bug.cgi?id=706005
MetaIdleMonitor: add a DBus interface for the idle monitor

To allow other clients (gnome-session, gnome-settings-daemon)
to monitor user activity, introduce a DBus interface for the
idle monitor inside mutter.

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