Massive changes for S3M compatibility
- settled to period indices in module structures
- period lookup stuff moved to specific loader and a generic lookup for ui
- new generic loader to load any supported module file
- protracker.h -> defs_mod.h (to correspond better with defs_s3m.h)
- current period table now located as pointer in player and set in player_set_module
  to transparently change the period table for each module type