Implement org.gnome.Mutter.IdleMonitor

The idle monitor is used (via GnomeIdleMonitor) in gnome-session to determine if a user session is idle and to export it's presence status (org.gnome.SessionManager.Presence) and screensaver (org.gnome.ScreenSaver) interfaces. The former is then used by gnome-shell to determin whether to lock the screen or not. If we want idle-inhibit to work via gnome-session we need to implement the above interface in phosh and instead of listening for idle timeouts directly act on gnome-session's presence status changes.