expose the id attribute of beam in `mx::api`

[webern]

MusicXML 3.1 gives many elements an optional id attribute. mx::api exposes the id of elements that have a matching mx::api type (#399). <beam> was left out.

Today the value is dropped. Reading a file loses it, and there is no way to author one.

NoteData::beams (src/include/mx/api/NoteData.h) is a std::vector<Beam>, and Beam is an enum. A beam carries no other data in mx::api, so there is nowhere to put an id.