[platforms/x11/standalone] Port to AbstractOutput
Represent outputs in the X11 session via AbstractOutput. For that we
move all Wayland specific parts of AbstractOutput into a new subclass
AbstractWaylandOutput and let the outputs of our Wayland backends inherit
This should allow us to get rid of the Screens class later on.
Test Plan: Manually in X session.
Reviewers: KWin, zzag, davidedmundson
Reviewed By: KWin, zzag, davidedmundson
Subscribers: ngraham, zzag, kwin
Differential Revision: https://phabricator.kde.org/D19208