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.
