Big update to the game loading and game playing

1. Move the game loading and game playing function into separated class.
2. Add game loading images into Images class.
3. User JPanel to draw the game loading and game playing stuff, use
paintComponent(Graphics g) instead of paint(Graphics g), be careful that
the parameter g in paintComponent is different from the parameter g in
paint, you need to use this.getComponentGraphics(this.getGraphics() to
get the g in paintComponent.
4. Update the enemy plane fighting and killed draw method.
5. Update enemy plane, bullet, my plane draw method.