rev1:
This is a first rough split-up of Compositor class. It works but there is
possible more functionality that could be put into one of the child classes
only.
rev2:
The `isCreated` function is only used in a single Q_ASSERT. I'm not in favor of keeping it. We should not need an assert for stuff like that. The currentRefreshRate extern function is there for no apparent reason.
rev3:
Move Suspend reasons into X11Compositor