[Trunk] Mixer doesn't show current instrument name after Change Instrument
1. Create score.
2. Right-click stave.
3. 'Stave Properties'.
4. 'Change Instrument'.
6. Open Mixer.
Expected result: The name of the changed instrument appears.
Actual result: The name of the originally-selected instrument is still there.
Using MuseScore 2.0 Nightly Build (4947) - Mac 10.6.8.