backends: Add logical monitor -> monitor -> output -> crtc ref chain

backends: Add logical monitor -> monitor -> output -> crtc ref chain

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1703668
Bug-GNOME: https://bugzilla.gnome.org/show_bug.cgi?id=786929
Forwarded: yes, https://gitlab.gnome.org/GNOME/mutter/merge_requests/81

Make it so that each logical monitor has a reference to all the
monitors that are assigned to it.

All monitors has a reference to each output that belongs to it.

Each output has a reference to any CRTC it has been assigned.

https://bugzilla.gnome.org/show_bug.cgi?id=786929

Gbp-Pq: Name backends-Add-logical-monitor-monitor-output-crtc-ref-chai.patch