Provide functionality of CompositingPrefs through Platform

Summary:
CompositingPrefs is only relevant for X11 standalone. It had some
"hacks" to make it not block Compositing on Wayland. Thus it was in
its current form not really useful.

Now all the functionality is provided through Platform with a default
implementation which is sensible for Wayland platforms.

The X11 standalone platform implements the new methods with the
Wayland checks removed.

In addition all calls to CompositingPrefs now go through the platform
directly and CompositingPrefs is completely dropped.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1576