Changes to Instrument names not reflected in "Parts"

Changing and instrument for a staff isn't takng into account by the "Parts" feature

Step to reproduce:
- Create a new 2 instrument score with two instruments (ie Flute and recorder)
- Change an instrument (ie. Recorder changed to Organ)
- Go to the "Parts..." dialog and the list of instruments still shows Recorder
- If we create the parts, Organ is never shown (in the in the score part, in the tab, etc).

GIT commit: e47d9de