Summary:
Replace the several internal state booleans of Compositor with a single
enum to register the current state of the compositor. We register four
states of starting, started, stopping and stopped.

The goal is to replace the several different conditionals when starting
and stopping the compositor with a single well defined flow.

There are currently still some ugly conditionals and some replaced ones
might need some more work.

Test Plan: Manually in X and Wayland. Relevant auto tests pass.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D22277