MIDI integration

At some point we'll need to map all the controls to work with MIDI controllers. It seems quite feasible: https://developer.mozilla.org/en-US/docs/Web/API/Web_MIDI_API.

We can provide defaults for most common controllers and allow to load a custom mapping.

Start by reviewing the Web MIDI API documentation linked in the issue and inventory the existing controls that need MIDI support. The work is complete when common controllers have defaults and users can load a custom mapping, but the issue does not identify files, entry points, or tests.

Written by the indexing model from the issue text.