[platforms/hwcomposer] Port to AbstractOutput

Summary:
This gives us XdgOutput support a requirement now that wayland server
reports that we support it and Qt5.12 then expects it to be sent.

It also provides an easy starting point for supporting wayland scaling
and a slight step towards multi-output.

Test Plan:
Ran on phone. Got output. Turning screen on and off works.
The double tap doesn't work great, but that was true before
(via bshah's hack to build on the current state)

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, nicolasfella, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18808