[platforms/x11/windowed] Port to AbstractOutput
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
Differential Revision: https://phabricator.kde.org/D19207