[xwl] Move Xwayland parts into separate class

Summary:
The Xwayland code path is moved from ApplicationWayland to a dedicated class
Xwayland in a new top-level directory xwl.

This is a direct preparation step for generic support of Xwayland Selections.

On a longer timescale this should also allow us to further separate Wayland
native functionality from Xwayland to allow us at one point to build KWin's
Wayland binary optionally without X dependencies. Another long term goal, that
becomes possible through this separation is to recover from Xwayland crashes.

Test Plan: Manually and auto tests

Reviewers: #kwin, davidedmundson

Subscribers: davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15020