Take vertical scrollbar into account when calculating navbar width

Summary:
Otherwise the vertical scrollbar obstructs the icons and forces a
horizontal scrollbar as well.

Test Plan:
Built and tested, no horizontal or vertical scrollbar
visible.

Reviewers: mlaurent, dvratil

Reviewed By: mlaurent

Subscribers: kde-pim, KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D5542