GBT Player v4.3.0

• GBA-only release.
• Refactor s3m2gbt.
• Refactor the C library.
• Implement event callback system.
• Add s3msplit to split S3M files into a GBT Player template file and a regular S3M file to be played by other regular S3M players.
• Add gbt_set_position() and gbt_get_position().
• Add examples, including one of using GBT Player at the same time as Maxmod.
• Fix code that disables channels in GBT Player.