Provide indexing in mx::api

[rpatters1]

As of now it seems like every client has to invent its own indexing to find items it has created previously. It would be nice if mx::api provided an internal index of elements, especially MeasureData, StaffData, and NoteData. Then clients could map these indices to their components on the other side.

This is just a wild-ass idea right now. I haven't really thought through any details. I am providing this kind of service in my mnxdom project, which is doing for MNX what MX does for MusicXml. But MNX makes it much easier, both by provided id values on every element in the spec, but also by being JSON. So I'm not sure what is involved for MX.