Fix bug #5408: tex2lyx cannot handle verbatim code

Fix bug #5408: tex2lyx cannot handle verbatim code

- Implement catcode setting in Parser
- add a new Parser::verbatimStuff method that reads verbatim contents
- use this method to parse "verbatim" environment.
- use it to parse \verb too.
- rename Parser::verbatimEnvironment to ertEnvironment.

TODO:
- use for other verbatim-like cases (Sweave chunk, lstlisting...)
- factor out the function that outputs ERT (including line breaks)
- maybe implement Parser::unparse (if needed)