#24 Cancel does not work when exiting application

[Petter W]

When exiting the application, there is a yes/no/cancel
question if the changes should be saved. If the user
presses cancel, the application in shut down, and saves
discarded. That is wrong, if cancel is pressed the
application should not be exited.

How to reproduce:
Start the application.
Add a series.
Enter something for book name.
In the file menu, choose exit.
In the dialog, choose cancel.