Window monitor-selection under Wayland

[Campbell Barton]

Recently the discussion on window-positions reminded me of an issue
that Blender is likely to run into when switching to Wayland
(currently work-in-progress).

While as far as I can tell window-positions aren't necessary - Blender
users frequently run with multiple-monitors and have windows dedicated
to a task.

Users *really* care about this [see links below],
if you're working with an animation time-line on one monitor and your
3D view on the other for example - having to arrange windows on
monitors every time you reload your file isn't nice.

An argument could be made that the compositor should detect this case
and handle it usefully,
(a single application creating multiple maximized windows for e.g.
could assign each to a separate monitor), although it seems likely the
window order wouldn't be maintained.
Checking Gnome-shell, multiple windows are assigned to different
monitors - but only when full-screen.

What would work well for Blender instead of absolute window
positioning would be a way to hint which head (monitor) to use.

The compositor would be free to ignore the hint, but in situations
where the number of physical monitors doesn't change, this would allow
users to re-open their projects and have the windows positioned as
expected.

----

Some bug reports relating to windows opening on the same monitor,
unrelated to Wayland, just an example that users find the
functionality important.

https://developer.blender.org/T34962
https://developer.blender.org/T34391
https://developer.blender.org/T36707
https://developer.blender.org/T42092

--
- Campbell