[KTextEditor] Port away from deprecated QFontMetrics::width
Summary:
Tried to choose horizontalAdvance() or boudingRect().width() correctly,
but it's sometimes hard to be 100% sure which one should be used.
Test Plan: Builds
Reviewers: cullmann, dhaumann
Reviewed By: dhaumann
Differential Revision: https://phabricator.kde.org/D23962