Make the journey search parser more generic

- SyntaxItem is now the base class for abstract match items, eg.
a sequence of items or a number.
- Matches in a given search string are represented by MatchItem objects.
- SyntaxItems can be easily combined with operators (| for options like
in regexps, + for sequences) and functions (eg. SyntaxItem::optional()
to make a syntax item optional). The syntax can then be defined easily
using the helper class Syntax, which is also easy to read.