refactor Boot.java to streamline game initialization and window generation

src/de/miaurizius/jgame2d/core/Boot.java

@@ -7,14 +7,21 @@ public class Boot {
 
     public static final Logger logger = Logger.getLogger("JDGame2D");
     public static JFrame window;
+    public static GamePanel gamePanel = new GamePanel();
 
     static void main() {
+        generateWindow();
+
+        gamePanel.setupGame();
+        gamePanel.startGameThread();
+    }
+
+    public static void generateWindow() {
         window = new JFrame();
         window.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
         window.setResizable(false);
         window.setTitle("JGame2D");
 
-        GamePanel gamePanel = new GamePanel();
         window.add(gamePanel);
 
         gamePanel.config.load();
@@ -24,9 +31,6 @@ public class Boot {
 
         window.setLocationRelativeTo(null);
         window.setVisible(true);
-
-        gamePanel.setupGame();
-        gamePanel.startGameThread();
     }
 
 }