Use ellipsis in search placeholder

Summary: Follow KDE Hig: https://hig.kde.org/style/writing/placeholder.html#actions

Test Plan: Build, and display search Search...

Reviewers: #kde_pim, mlaurent, dvratil

Reviewed By: #kde_pim, dvratil

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20054