Make Compositor::finish more generic

Summary:
KWin is not only an X11 compositing window manager, but also a Wayland
compositor. The Compositor class is used in both cases so in general it
would be nice to keep it as much as possible generic.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19879