[kcmkwin/compositing] Use ellipsis in Desktop Effects kcm

Summary: This patch introduces the ellipsis used in Krunner, Kicker, Kickoff and Clipboard to Desktop Effects (Kwin).

Test Plan:
Before: {F6522181}
After: {F6522182}

Reviewers: ngraham, #vdg, #kwin, davidedmundson

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

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D17937