Move quit event handling to the interface and flatten the Action class
hierarchy. There are no longer any actions that don't take an Actor. This has
the happy side effect of resolving some pylint errors too. :)
Events start in the Interface. The interface gets first crack at any incoming
events. If the interface doesn't handle the event, it is given to the
engine. The engine has an EngineEventHandler that yields actions just
like the event handler prior to this change.
The interface's event handler passes events to each window in the
interface. Windows can choose to handle events however they like, and
they return a bool indicating whether the event was fully handled.
