Consider adding trigger output in MIDI MONO mode

[ngwese]

The MONO mode as with the other MIDI modes only output gate signals. Unlike the other modes MONO mode does not use all the trigger outputs. TR1 is gate and TR4 is clock.

Using TR2 for trigger out on each note on would be a nice enhancement. Implementation would require setting up a timer to take TR2 low at some fixed duration after note on.