partial game logic separation

Demo:
-removed score, aircraft_waiting_message
+removed initialisation of score in start()
+removed airport waiting scoring/message from update()
-removed orders_box
+removed initialisation in start()
+removed update() call
-time_elapsed removed
+getter now non-static, delegated to GameWindow
-removed airport
-misc. comment tidies

GameWindow:
-changed existing and new naming conventions to match Java style
-reordered functions to try and give some semblance of meaning
-added score, shownAircraftWaitingMessage
+added update() call
-added OrdersBox instance, orders
+initialised in setAreas()
+added update() call
-added timeElapsed
-added airport
-added update()
+added check for aircraft waiting to take off
-added skeleton draw()

Main:
-added call to GameWindow.start()