Monophonic mode on instrument plugins; portamento

### Enhancement Summary
Add a monophonic mode to all native instrument plugins, (and possibly the VST/SF2 plugins too?)
Followed up, possibly, by a portamento option. (Possibly a knob too, that would be cool, for portamento amount.)

#### Justification
Monophonic mode would be useful.
About portamento: As of now, only the LB302 has a portamento option because the synthesizer originally was monophonic and had portamento.
However, a lot of the time we want a portamento sound with the native plugins, and pitch bending every note to this end is hard. (Perhaps it would be better to be able to pitch bend all native instruments before adding a portamento... I'm not so sure of this, though.)
I don't want to have to use ZASFX or an external VST each time I want one-click portamento.

As an afterthought I'd like to mention I've seen the Kicker being used in ways that can produce long notes (see Jousboxx, Buzzer Beater), and guitars can have glide, though very short- (I'm looking at Vibed). So I generalize all instruments, despite how it might be sensible to exclude ones with a percussive envelope.

Start by surveying the native instrument plugins and the existing LB302 portamento behavior. Clarify whether VST/SF2 plugins are in scope and whether monophonic mode, portamento, and a portamento control are separate deliverables; done should be defined per plugin before implementation.

Written by the indexing model from the issue text.