user added accidental stays when note moved using UP/DOWN keys

1. create a new piano score
2. enter quarter notes: c b DOWN a b
3. click on the 'flat' symbol to add a flat to the last note
4. go back to the second note and press UP
5. go to the last note and use the UP/DOWN keys on it.

Expected result: the flat in front of the note to be replaced as needed
Actual result: the flat stays whatever pitch the note becomes

Using 2d506f3 on Ubuntu 12.04

This might be a problem with the fact that Note::_accidental is always undefined.
In Score::changeAccidental() line 1302 for example it is used to find such an user added accidental.