KCM/Compositing: Use KConfig XT in UI

Summary:
Use KConfig XT to manage most fields of the KCM.
Simplify code.

Depends on D27955

Test Plan: No functional change

Reviewers: #kwin, ervin, crossi, bport, hchain, zzag

Reviewed By: #kwin, ervin, bport, zzag

Subscribers: anthonyfieroni, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27988