Change a couple instances of QFontMetrics::width()

Change a couple instances of QFontMetrics::width()

Use QFontMetrics::boundingRect() instead. QFontMetrics::width() does
not actually calculate the width of the bounding box and is mostly
useful for knowing where to draw text after a string, taking into
account bearings. Indeed, it has been renamed to
QFontMetrics::horizontalAdvance().

For the code touched in this commit, we want to center a string.
Either method would likely be fine, but it is more easy to
understand exactly what QFontMetrics::boundingRect() does.

This commit does change functionality, although it should not be
noticeable.