Initialize Wayland output in AbstractOutput

Summary:
Wayland output is on protocol level and not dependent on the hardware
platform. Next steps are to do the same for output device and then let
the virtual output call into these initializing functions as well.

Test Plan: Manually and auto test.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16783