Turn popover activation button 'on' when the popover is up.
Remove mappingsChanged with a notification. MappingsController no longer needs to know about the app delegate.
Tweak toolbar icons.
Add some categories to better handle menus with represented objects, then use them for simpler menu handling.
Support sudden and automatic termination.
Avoid repeating serialization codes.
Subscribe the output controller to notifications directly. Application delegate has no more need to track the output controller.