Update search bar following kde HIG
Summary:
- Use ellipsis and placeholder
- Extend search field
Screnshots:
Test Plan: Compile, run and work like before
Reviewers: Cantor, VDG, asemke
Reviewed By: Cantor, asemke
Subscribers: ngraham, aacid, kde-edu
Tags: KDE Edu
Maniphest Tasks: T10258
Differential Revision: https://phabricator.kde.org/D19773