Notify on notebook change for exceptions.

If you set (or change) the notebook of a recurring event with exceptions, the notification of change is emitted only for the parent event, but not for the children. If you change to a notebook that is hidden for instance, the signal is missing for the children and they are kept visible while the parent becomes hidden.

I've added a test to check that the update is properly emitted when a notebook is changed.

@gditchfield, @dfaure or @winterz , if you would like to review this MR, please. I thank you in advance.