refactor: use setter - give GameState a *temporary* ref to GameActor

This reference in GameState can be removed once it is given to AIPlayer in game
initialisation (setupGameState)

app/actors/GameActor.java

@@ -65,7 +65,7 @@ public class GameActor extends AbstractActor {
 		// Initalize a new game state object
 		gameState = new GameState();
     // #hack: give the game state a reference to the game actor so AIPlayer can send messages
-    gameState.gameActor = this;
+    gameState.setGameActor(this);
 		
 		// Get the list of image files to pre-load the UI with
 		Set<String> images = ImageListForPreLoad.getImageListForPreLoad();