Add XDG Output support

Test Plan:
Very minimal expansion of unit tests which uses WaylandScreens
Wrote mini app to debug actual output of xdg-output for testing the DRM code

Main relevant user of this is xwayland > 1.20 which I don't have, so that
part remains untested

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: romangg, graesslin, bshah, kwin

Tags: KWin

Maniphest Tasks: T8501

Differential Revision: https://phabricator.kde.org/D12243