[MU3] Labeling solo parts

[njvdberg]

Resolves: https://trello.com/c/piTSKoq2/28-solo-instrument-names-should-end-in-solo-eg-flute-solo-soprano-solo

Solves two issues in two different commits:

1. When creating a new score, Soloists wasn't store in the part.
2. New instrument numbering, renumbers instruments and soloists independently.

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