It is not practical, as it is not easily possible to nest parsers.
This fix stops unquoting strings in lexer, and postpone it to syntax tree
walking code.
Add tests showing search key nesting works with different unquoting rules.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@40389 20353a03-c40f-0410-a6d1-a30d3c3de9de