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

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

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.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=786929
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/1703668
Forwarded: yes, https://gitlab.gnome.org/GNOME/mutter/merge_requests/81
Applied-upstream: 3.28.3, commit:fe1616668e9d82e87f6c518b3b4bacff3afefdf7

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