Status bar mode menu: Reuse empty QIcon that is implicitly shared

Summary:
Instead of creating N empty QIcons, use just one QIcon
that internally is implicitly shared, see:
https://doc.qt.io/qt-5/implicit-sharing.html

Test Plan: make

Reviewers: cullmann

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D24166