[effects] Move Fade, Glide, and Scale effect into exclusive group

Summary:
Fade, glide, and scale effect are mutually exclusive effects so they have
to be put into an exclusive group in the desktop effects kcm.

Test Plan: {F6418669}

Reviewers: #kwin, #plasma, #vdg, ngraham, graesslin

Reviewed By: #kwin, #plasma, #vdg, ngraham, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16858