Use ellipis in search placeholder

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

Test Plan: Compile, placeholder is now Search...

Reviewers: KDE PIM, mlaurent

Reviewed By: KDE PIM, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20053