[MathML4] thin, medium and thick values for mfrac@linethickness

Hi,

The mfrac elements has values "thin", "medium" and "thick" which are
"left up to the rendering agent". This is prone to inconsistencies
between implementations. During Igalia's MathML refactoring, we aligned
WebKit on Gecko's behavior. This is essentially what is suggested in the
MathML in HTML5 implementation note and that should probably be included
in an official MathML recommendation too:

* The default linethickness "medium" is given by the
FractionRuleThickness constant of the MATH table of the font used for
the MathML rendering (see
https://lists.w3.org/Archives/Public/www-math/2016Jul/0025.html for
discussion about how to determine that font).

* "thin" is 50% and "thick" is 200% of the default linethickness (these
values are arbitrary and based on how it has been implemented in Gecko
probably for more than 15 years).

But are these keywords really used in practice? If not maybe you may be
inclined to deprecate them too.

Frédéric