Use virtual ports in configuration/expressions

[ryanlaws]

Summary

As described in #2, MIDI are currently expressed as port IDs. Let's not. Human-readable names are nice. Platform-supported identifiers are just as good. Support the virtual ports (the 4 you select in the system menu).

Why

These are a well-established platform idiom and we should use them. The norns platform already provides a good UI for selecting these and that saves us work - we get port selection for free, users will "get it", and that's a UI problem we don't need to solve.

Considerations

• A plisp function to return a low-level port number based on a virtual port number would be fine, and probably the easiest approach based on where the ball lies today.
• Refactoring other functions to use virtual ports is also acceptable. Note that the ports identified in #2 do not necessarily overlap with these - there will only ever be 4 of these and there can be many more low-level ALSA ports.

[ryanlaws]

This PR from @dndrks adds support for 16 ports instead of 4, effectively removing any reason to use non-virtual ports: https://github.com/monome/norns/pull/1268