Add doctest for handling the 'new_game' keypress
Refactor key bindings into key_binding_manager

Pass the KeyBindingManager from the Controller into the View instead of
passing the key_map directly.
Add model as a Controller test dependency
Change KeyBindingManager.get_key to return None if there is no corresponding key
Separate the action lookup from the action handling

* Have KeyBindingManager keep a key-to-action map in addition to its
action-to-key map.
* Move the Controller._handle_event doctests to the top-level Controller
doc-comment.
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