Make ID attributes available in mx::api. #399

Beginning with MusicXML 3.1, many MusicXML nodes can have an ID attribute. mx::api should make this available. This is related to, and requested in discussion #349 which lead to (the challenging to design and implement) issue #393.

In the discussion @rpatters1 said:

As an aside, if ID values were available in mx::api, I would be using them a lot more right now.

This is very much in this library's wheelhouse. A bit more complicated is the related #397. But with-or-without uniqueness validation, adding attribute fields that mx::api is agnostic about is easy.

Edit:

#399 exposes the most easily reachable IDs, but the following require more research or design:

• expose the id attribute of dynamics, fermata and accidental-mark in mx::api #403
• expose the id attribute of notations, articulations, ornaments, technical and direction-type in mx::api #404
• expose the id attribute of beam in mx::api #405
• expose the id attribute of harmony and frame in mx::api #406
• expose the id attribute of credit in mx::api #407
• expose the id attribute of print and measure-style in mx::api #408