Instrument change in subpart doesn't change the master score

When working on a subpart of a score, if we change the instrument, that change doesn't reflect on the master score. But this should work the same way has when adding notes to the staff.

Step to reproduce :
- Create a multi-intruments score
- Go in "Parts", click "New all" and Ok
- Click on a tab for an instrument.
- Change that instrument (right clicking the instrument name)
- Go back to master score, no change.

GIT commit: e47d9de