InstrumentMidiIOView: added support for fixed output notes

It came to me that having LMMS output one fixed note from a track could
be useful for controlling drum machines or something like that, so here's
a new spinbox for the MIDI tab.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f