Display footnote labels as superscripts

Display footnote labels as superscripts

A new counter type \superarabic is introduced, that transforms the
counter into superscript Unicode numbers.

This is used for the footnote counter.

The Foot inset layout is modified to use an empty LabelString (in size
Normal, since the numbers could be too small).

We use this occasion to set footnote text in footnotesize, as it should.

A couple of changes are done to the code to make the button label and
tooltip look better.

Fixes bug #12238.