[NEW] Numbering od slides in slideshows

[Rob Beezer]

Slideshows (Reveal, Beamer) can now number slides. These are PreTeXt numbers -
we have taken over these tools' own ideas about numbering, in part for consistency.

Level 0: 1 -> N, just on #slide (not a title page, etc)

Level 1: numbers like M.N, the Nth #slide of the Mth #section

* I have killed the total number of slides for Beamer ("5/97") since I think
you never want your audience to know how close you are to the end. Or how far
from the end. ;-)

* Beamer themes are all over the map - report any themes where numbers
mis-behave. We've checked a dozen or so. Could become a case of whack-a-mole,
but maybe not.

* The Reveal numbers are rather small, making them larger could interfere with
the page controls, but we could look at it.

* There have been a couple fixes related to numbering blocks and numbers in
#xref as part of this work.

* I found an earlier iteration of this useful when I made embossed braille
slides for an audience member who is blind (not sure this is currently
supported, it was an experiment). I was able to read slide numbers as as I
advanced on the screen and the braille had the same numbers on each slide.

https://github.com/PreTeXtBook/pretext/pull/3237

Subsection 45.6.1: Reveal.js Appearance
https://pretextbook.org/doc/guide/html/publication-file-revealjs.html#revealjs-appearance-options

Subsection 45.7.1: Beamer Appearance
https://pretextbook.org/doc/guide/html/publication-file-beamer.html#beamer-appearance-options

Rob