Port from QFontMetrics::width() to boundingRect().width()

Summary:
In all the cases that were changed, the bounding rect is needed. A few
places were not ported, since they need more careful investigation.
This reduces deprecation warnings.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D23144