Added revalidation scheme to NoteData's all_tracks_iterator so that it can be made valid again after the NoteData is transformed by something adding or removing steps.  This fixes the crash on courses that use transform mods on the second song.  ITG2's Getting Started is an example.
Merge pull request #236 from kyzentun/RevalidateNoteDataATIs

Added revalidation scheme to NoteData's all_tracks_iterator