Refactoring LaTeX conversions

[Rob Beezer]

* With my assistant, we have done a big refactor so that
"pretext-latex-common.xsl" really looks more like a set of "common" templates to
be shared across the LaTeX conversions: "regular", classic, and Beamer. For
Oscar: the "pretext-classic" stylesheet was not touched in this branch/PR.
https://github.com/PreTeXtBook/pretext/pull/3017

* The LaTeX conversions define a handful of "internal, semantic macros" (well,
66 of them it seems). \mono{} is a good example. Just a useful device to
isolate definitions in the LaTeX preamble. Our \alert, was conflucting with
Beamer's "theme-aware" \alert. So we undertook the long-mooted crude
"name-spacing" of prefixing these macros with \ptx. So we now have \ptxmono and
\ptxalert. The latter allows us to defer to Beamer in this situation.

These macros are not a public API, so we can (and just did) change them at will.
But perhaps you have messed with them via some addition to your preamble? So,
the courtesy of a heads-up.

For Oscar: a few cosmetic changes
https://github.com/PreTeXtBook/pretext/pull/3015


@Oscar - I'm going to send you some questions off-list, since they barely make
sense to me and likely won't add any value here publicly.

Rob