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