Add Track info to MIDIMessages

[Sammy1Am]

Adding track info to MidiMessages would allow message routing based on tracks instead of channels (so that you could compose MIDI files for more than 16 individual instruments). However the current implementation of RealTimeSequencer and MidiMessage doesn't pass any track number information via the MidiMessage (i.e. once the message has reached the sequencer we have no idea which track it came from).

Adding support is technically easy (add a trackNumber field to the MidiMessage class and populate it in the RealTimeSequencer), but practically difficult as it would mean needing to reimplement the RealTimeSequencer and the current source license isn't compatible with the goals here.

If anyone happens to know of an alternative implementation of a MIDI sequencer in Java (ideally licensed under the Apache or MIT licences) it might be a lot easier to leverage that code to replace the default Java sequencer.