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. :)
The base class Actor doesn't declare a (optional) actor attribute.
The ActionWithActor has a non-optional actor attribute.
This makes the type checker happier, and means we can have some actions
that don't have actors.
Most of the existing actions are game actions (they control the player character)
so they live in actions.game. Eventually, there will be modules for different
kinds of actions that only apply to, e.g. modal UI.
