Split Compositor class in Wayland and X11 child classes

Summary:
This patch is a first take at splitting up of the Compositor class into
Wayland and X11 child classes.

In this first patch we mostly deal with setup and teardown procedures.
A future goal is to further differentiate the compositing part itself too.

Test Plan: Manually X from VT and Wayland nested. Autotests pass.

Reviewers: #kwin

Subscribers: sbergeron, anthonyfieroni, zzag, kwin

Tags: #kwin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22195