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* Adjusted the `SS_GROUPBOX` stylesheet: Groupbox title is now vertically
aligned to be centered with the top line, regardless of the Qt lib. Nothing
changes for PyQt5/PySide2, and fixes PyQt6/PySide6.
* Added function `e8()` to calculate the pixel width that is necessary to fit
`N` number of '8's in the supplied `font`. In analogy to the 'em' font-metric,
but now focused on displaying numeric values.