Increasing rest value causes corruption

Context: commit b87ba9646

Steps:
1) Create a new score
2) Select one rest (for the moment, there are only whole measure rests)
3) Change it into a quaver rest
4) Change the quaver rest into a crochet rest

Result: the measure contains the wrong total duration: one of the quavers disappears (correctly) but the other does not change into a crochet.

More steps:
5) Undo: the measure spacing changes, but there is no change to the rests
6) Undo again: the whole measure becomes empty

Note: In general, the problem arises when the rest value is increased from any value to any greater value, but not when it is decreased. While increasing, the rest may 'eat' a following rest or note, it makes no difference: corruption arises in both cases.

M.