Add a highlight grid to Map, with locations set to True if that point should be
drawn with a "highlight" treatment.
Add the highlight graphic_dtype to all Tiles.
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.
- Factor a bunch of event handling into a base EventHandler class
- Rename the previous event handler MainGameEventHandler
- Add a GameOverEventHandler that only responds to Exit actions
