Summary:
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
from there.
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
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D19208
