Percussion mappings

[adrianholovaty]

How can MNX encode a percussion mapping? This would be a part-specific data structure that answers the following questions:

• Which percussion sound should be used for a given combination of staff position + notehead type?
• Given a percussion sound (e.g., "Hi-hat open"), which staff position and notehead type should be used by default?

The goal would be to encode music in a semantic and flexible way, so that the document isn't redundantly repeating this information and ideally decisions can be changed in a single place. Perhaps a note object would have a reference to a specific ID in the percussion mapping, as opposed to having a hard-coded pitch or staff position. And of course it would be good to allow for per-note overrides.

I don't necessarily think this work is a huge immediate priority, but I'm just creating an issue to track the TODO.

#316 (encoding unpitched notes) is very closely related. From my perspective, that issue is more about encoding ad hoc unpitched notes in the middle of a pitched instrument's music, as opposed to encoding an entire instrument's worth of unpitched notes. For the latter, a percussion mapping is a much better solution.