Place doors on the level

Pick a random subset of cells on the level that have a wall on two
opposite sides and are clear on the other two sides, and place closed,
unlocked doors on those cells.

src/Xanthous/App.hs

@@ -90,11 +90,7 @@ initLevel = do
     generateLevel SCaveAutomata CaveAutomata.defaultParams
     $ Dimensions 80 80
 
-  entities <>= (SomeEntity <$> level ^. levelWalls)
-  entities <>= (SomeEntity <$> level ^. levelItems)
-  entities <>= (SomeEntity <$> level ^. levelCreatures)
-  entities <>= (SomeEntity <$> level ^. levelTutorialMessage)
-
+  entities <>= levelToEntityMap level
   characterPosition .= level ^. levelCharacterPosition
 
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