* Add percentage to FSRS spinner
This commit add a percentage option in SpinBox and SpinBoxFloatRow, set to False
by default.
If it's true, a percent symbol is added at the end of the line before
the increase/decrease button.
While the value is represented as a percentage without decimal places,
the internal representation is not changed. Which mean that a
multiplier must used to compute the string value, indicate to the
input field the min, max and step, and when updating the result.
* Remove unsightly percentage sign, and update historical retention too
https://github.com/ankitects/anki/pull/3679#issuecomment-2579636981
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Co-authored-by: Damien Elmes <gpg@ankiweb.net>