[effects] Put Window Aperture and eye On Screen in an exclusive category
Having these two enabled doesn't make sense. Also, Window Aperture + eye
On Screen looks really weird.
Opened the Desktop Effects KCM, both the Window Aperture effect and the
eye On Screen effect are in an exclusive category:
Subscribers: ngraham, davidedmundson, kwin
Differential Revision: https://phabricator.kde.org/D14456