Generate settings pages automatically.

This adds metadata to settings fields that can be used to automatically
generate the settings window. For now I have replaced the Difficulty
page. Will follow up to replace the others.

game/settings/__init__.py

@@ -0,0 +1,5 @@
+from .booleanoption import BooleanOption
+from .boundedfloatoption import BoundedFloatOption
+from .choicesoption import ChoicesOption
+from .optiondescription import OptionDescription
+from .settings import AutoAtoBehavior, Settings