Set tablature string assignments via String Number objects

[worldwideweary]

First part of the request from scorster musescore#22228 including his discussion post: https://musescore.org/en/node/362557

I'll link a PR for testing if that's cool. Something simple that just takes into account the string number without any options or anything. No Capo implementation (yet). That way I can get some feedback if there's something not working etc.