Summary:
To homogenize our backends and as another step to remove the Screens class
use the AbstractOutput class in the windowed X11 backend.
Test Plan: Manually in X session.
Reviewers: #kwin, zzag
Reviewed By: #kwin, zzag
Subscribers: davidedmundson, zzag, kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D19207
