MusicXML: Implement part-group (i.e. StaffGroup in ly) and part-name (instrumentName)

MusicXML: Implement part-group (i.e. StaffGroup in ly) and part-name (instrumentName)

Implementing part-groups and their bracketing and span-bars turned out
harder than I imagined... I build one nested set of StaffGroup and Staff
objects from the part-list, including the instrument names and the bracket
types. However, to be able to distinguish single-staff staves and piano
staves and to assign the correct voices and lyrics to a part, I need
another loop after everything has been processed. In this additional
loop through the whole score structure I extract the voice and lyrics
ids assigned to each part and assign them the StaffGroup nested
objects.

Finally, printing everything out is quite simple.

Signed-off-by: Reinhold Kainhofer <reinhold@kainhofer.com>