- make FileIndex const correct (std::map::operator[] modifies the std::map)
- use a single map
- keys are paths relative in the game data directory + filenames
(so paths like "data/main.scm" can be found twice in the map)
- normalization of the file paths is done inside FileIndex,
so there shouldn't be any ::tolower's anymore.
- added a normalizer helper function "normalizeFilePath"
- added Documentation
