Feature: Decoder for DOS version music

This is based on reverse-engineering the TTD DOS driver for General MIDI music.

src/music/midifile.hpp

@@ -17,6 +17,8 @@
 #include "midi.h"
 #include <vector>
 
+struct MusicSongInfo;
+
 struct MidiFile {
 	struct DataBlock {
 		uint32 ticktime;           ///< tick number since start of file this block should be triggered at
@@ -35,6 +37,8 @@ struct MidiFile {
 	uint16 tickdiv;                  ///< ticks per quarter note
 
 	bool LoadFile(const char *filename);
+	bool LoadMpsData(const byte *data, size_t length);
+	bool LoadSong(const MusicSongInfo &song);
 	void MoveFrom(MidiFile &other);
 
 	static bool ReadSMFHeader(const char *filename, SMFHeader &header);