Really fix bug #4468.

Really fix bug #4468.

The old fix was incomplete (\verb~\~ was translated to \verb~~ in roundtrip).
The real cause for this bug (and also the mistranslation of \href{...}{\}})
was the misbehaviour of Token::character() (see comment in Parser.h): This
method even returns a character if the category is catEscape, and this is not
wanted in most (all?) cases.