Abstract OutputDevice initialization

Summary:
Move generic Wayland parts of OutputDevice initialization into
AbstractOutput class.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Maniphest Tasks: T10016

Differential Revision: https://phabricator.kde.org/D16789