Fix build after deprecated QFontMetrics::width()

Summary:
It was deprecated for not reporting the actual width but the
horizontal advance. Use the actual bounding width instead and
don't claim to support future deprecations.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D11117