rpn item: removed index member and changed text generation

rpn item changes:
- removed the index member, which was only being used for testing output
- modified attached member access functions (made constant as they don't
  modify contents, and made attached function return the entire vector
  instead of just one element)
- removed text function (now handled by rpn list text function)

rpn list text function changes:
- now handles generating the strings of the attached rpn items so that
  it can generate the indexes for the attached items (the indexes are
  temporarily assigned to each item in the list and held in a standard
  unordered map
- the final string is now built up using a standard string stream, which
  for now is converted to a QString for returning (this will be changed
  to a standard string at some point along with the token text function)