Fix some texl2yx accent bugs

Fix some texl2yx accent bugs

This fixes bug #8554 and some recently introduced busg:
- Encodings::fromLaTeXCommand() can now handle all combining characters,
  not only the one letter ones
- The remainder returned from Encodings::fromLaTeXCommand() must never be
  thrown away in tex2lyx, but output as ERT
- No special case for combining diacritical marks needed anymore in parse_text()
- No special cases for accents and IPA combining diacritical marks needed
  anymore in parse_text()
- special tipa short cuts may only be recognized if the tipa package is loaded
- Use requirements returned by Encodings::fromLaTeXCommand() instead of
  hardcoded registering of tipa and tipax
- Get rid of the name2 variable in parse_text(): We must use name, otherwise
  the extra stuff that might have been put into name vanishes