[kcmkwin/compositing] Hide config options which don't make sense if Compositing is required

Summary:
On Wayland compositing is required. It doesn't make any sense to offer
the user to disable the compositor or to allow windows to block
compositing.

The Compositor DBus interface gains a new property whether the platform
requires compositing. This is queried by the compositing kcm and based
on that the two mentioned options get hidden and not stored to config
file.

Test Plan: Opened the KCM on Wayland

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2586