Use ellipsis in search field

Summary: Screenshot:

Test Plan: Build, run and search field work as before

Reviewers: Kalzium, VDG, yurchor

Reviewed By: yurchor

Subscribers: aacid, yurchor, ngraham, kde-edu

Tags: KDE Edu

Maniphest Tasks: T10258

Differential Revision: https://phabricator.kde.org/D19774