[kcmkwin/decoration] Add ellipsis to window decoration search

Summary: This patch adds an ellipsis to the search function in Window Decorations.

Reviewers: KWin, VDG, Plasma, zzag, ngraham

Reviewed By: KWin, VDG, Plasma, zzag, ngraham

Subscribers: kwin

Tags: KWin

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D18050