[kcm/effects] Clip ListView

Summary:
Otherwise the content overflows the frame when scrolling.

QQC2 scrollview docs say "ScrollView does not automatically clip its contents. If it is not used as a full-screen item, you should consider setting the clip property to true"

Test Plan:
Before:

After:

Reviewers: KWin, Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D27558