[effects] Put Window Aperture and eye On Screen in an exclusive category

Summary:
Having these two enabled doesn't make sense. Also, Window Aperture + eye
On Screen looks really weird.

Test Plan:
Opened the Desktop Effects KCM, both the Window Aperture effect and the
eye On Screen effect are in an exclusive category:

{F6160394, layout=center, size=full}

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

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

Subscribers: ngraham, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14456