Dissociate Staff name from Instrument

When creating staves, they're named after the name of the instrument, which is normal an whishable.

When creating parts from a score, they're situations when you don't want that instrument name be reflected into the staff name.

Examples:
- when you make an instrument change for clap hands
- when you make compound parts for the score (ie. using voices for instruments, say, Horns for example) and dissociated ones for the parts (ie one staff per voice (say, Horn 1, Horn 2, etc)
- when you've carefully named all you staves (Say, for example, flute 1, flute 2 and piccolo) and you want to change the instrument without changing this name.

So it'll be nice to can:
- name "staves" instead of "instruments"
- name differently a staff in the score and in parts