[kcmutils] Add ellipsis to search labels in KPluginSelector

Summary: This patch adds an ellipsis to the search labels in KPluginSelector (and the elements that call for it, e.g. KWin Scripts).

Reviewers: VDG, Plasma, kde-frameworks-devel, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, pino, shubham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D18043