Add general data file functions
Created by: christopho
Data files can be opened from the data directory/archive of the quest, or from the quest write directory. We need Lua functions to make this feature usable.
- Replace sol.game.exists() by sol.file.exists()
- Replace sol.game.delete() by sol.file.remove()
- Create sol.file.mkdir()
- Create sol.file.open() that returns a file. The file is opened from the appropriate place with io.open. This "appropriate place" can be determined with PHYSFS_getRealDir().