Moving instruments up or down through instrument list is tricky

1. Open demo score "bach-bc2" (or another score with more than three instruments)
2. Create > Instruments...
3. Click on "Trumpet" to select it
4. Click on "Down" to move the trumpet part down the list
5. Click on "Down" again

Expected behavior: Trumpet moves down to become third instrument from top

Actual behavior: Trumpet moves back up to top

(First reported by TKoorn: http://musescore.org/en/node/1309 )