Fix QFontMetrics::width() deprecation warning by switching to boundingRect().width()

Summary:
According to an article by @cfeck [1], QFontMetrics::width() gave the horizontal advance, which doesn't necessarily include every pixel that the text might use.
He suggested that boundingRect().width() might be what most people actually want.

- [1] https://kdepepo.wordpress.com/2019/08/05/about-deprecation-of-qfontmetricswidth/

Reviewers: #breeze, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: hpereiradacosta, cfeck, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26270