[platforms/hwcomposer] Port to AbstractOutput
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.
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
Differential Revision: https://phabricator.kde.org/D18808