Can't redo certain commands after undoing a "multiply tempo"

[rozlynd]

Starting from some sequence:

1. Multiply tempo (by 2 for example);
2. Add a note on one of the empty lines just created;
3. Undo twice to remove the note and restore the original tempo;
4. Redo the Multiply Tempo;
5. Redo again: the note is not re-added like it should.

This is just an example. It happens because the Command Manager stores information that may become invalid as a result of some undos. In this case, the line in which a note should be re-inserted has been removed from the sequence.

[rozlynd]

What I really want is to make some structures immutable, so all the editing has to go through some specific class that has access to the sequence's data (maybe StaffSequence itself). This problem will have to be resolved in the process.