Create a bandtrack for MIDI files by picking out the MIDI 0xCn events, and creating matching DMUS_PATCH_PMSG.
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This also has a commit adding a stub chordtrack, which is small so I think a separate MR is unwarranted. Can be split out if necessary.
-- v4: dmime: Add a stub chordtrack for MIDI segments. dmime: Parse MIDI program change events and generate a bandtrack. dmime: Better MIDI parsing interface. dmband: Move band.c to dmusic