Don't deal with adding/removing tiled Xrandr monitors in the generic
backend, but leave it to the Xrandr backend. The tiled monitor will
itself notify the backend when such a monitor is added and removed.
Tiled Xrandr monitors are now based no MetaMonitor instead of
MetaLogicalMonitor. This means that mirrored tiled monitors will now be
represented correctly.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
Instead of using crtcs and outputs to generate logical monitors, use
the ready made monitor abstraction that hides irrelevant things such as
monitor tiling etc.
https://bugzilla.gnome.org/show_bug.cgi?id=777732
