Use QFontMetricsF over QFontMetrics #6230

We currently use QFontMetrics, but we should use QFontMetricsF instead as stated in the docs:

QFontMetrics functions calculate the size of characters and strings for a given font. The class is an integer-based version of QFontMetricsF and will round all numbers to the nearest integer. This means its results will be inaccurate for any font with fractional metrics. In most cases QFontMetricsF should be used instead.

This might be a bit tricky as we use integers for most things.