[platforms/x11/windowed] Port to AbstractOutput

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