Add hardware control for Fluidsynth portamento

[Ekolide]

Hi! There's no portamento enabled by default on the Haxophone as of yet. Fluidsynth supports this through MIDI CC 5 (and I think also CC 37), I would love to see portamento control in the same vein as the current Soundfont preset control. Except instead of incrementing and decreasing the portamento with no user feedback, the Haxophone could offer twelve different levels of portamento, from a low D being the least portamento, to the C# right below the octave being the most portamento.

Rummaging through the Fluidsynth documentation, there's also a "Breath Mode" to get MIDI Breath CC 2 to control volume instead of CC 11. I'm not sure how the Haxophone implements the volume changes right now, and most EWI players I've seen prefer breath to control the filter anyways. But I thought I'd mention it.

EDIT: Thinking some more, I realize this is too much. Rather than hardware control, any kind of portamento control at all is more important than realtime editing.