Change instrument - creating new staves

Steps to reproduce bug
1. Create score for Violin
2. Right-click on an empty part of the staff, choose Staff Properties
3. Switch to Piano
4. Press "OK"

Result: Staff changes to Piano
Expected: New staff is created, one with the treble clef, and the other with the bass clef.

There are a few things to decide though:

1. When changing from a 1 to 2 staff instrument, where should the melody go? I would propose that it should go to treble clef if the 1 staff instrument had a clef higher than the C-3 clef.

2. When changing from a 2 staff to 1 staff instrument. This would involve removing one of the parts. I would propose something similar to the first solution. Leave the staff with the clef similar to the new instrument clef.