[platforms/x11/standalone] Port to AbstractOutput

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