[kcmkwin/compositing] Show warning message when selecting "dangerous" options

The idea is to warn the user that changing the settings might break
the setup when we know it could be dangerous. The following cases are
considered:
* Scale filter Accurate
* Tearing prevention
* keep window thumbnail always
* EGL if EGL and GLX are available
* unredirect fullscreen not working on all hardware

In addition the OpenGL is Unsafe warning is turned int a KMessageWidget
which is also used for all the other warnings.

REVIEW: 118494