Improve doxygen highlighter...

- Correctly terminate style commands the same way Doxygen does, i.e. highlight '@c foo.' correctly.
- Support multiple style commands, e.g. '@c @e foo' (note: right now the patch to make Doxygen itself treat these correctly is still outstanding)
- Better highlighting of @param
- More correct support for @par (move from TagWord to TagString)
- Recognize @cond/@endcond
- Recognize Doxygen groups