correctly calculate fontmetrics

Harald Sitter requested to merge work/advance into master
  • use the fontmetrics of the target label not the application (makes no difference but is practically more correct)
  • use horizontalAdvance instead of boundingrect width. the latter is relative to the rect coordinates and as such not nearly as straight forward to use. horizontaladvance is the actual width the text will occupy, it's what we intend to know here

BUG: 431447

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