Fix html export of \ne (second part of bug #9372)

The fix consists of two parts: The needed infrastructure to assign HTML
entities to global math macros, and the actual fix for \ne and \neq.
Store ASCII data in std::string

Math inset names and LaTeX requirements are always ASCII. This avoids some
conversions and makes math requirements more similar to text requirements.
Skip missing optional argument insets in tex2lyx

Optional arguments are optional, so if one is missing, do not stop the parsing,
but continue with the next argument (it might be a required one).