Rewritten Screen code to make it more like HOU's (fixes #12)

- This is a big commit which moves the stage handling code to AbstractScreen
- This also moves the screen changing code to ATC which was converted to
  a Game class
Merge remote-tracking branch 'github/master'

Conflicts fix handling of the ATC.showEndScreen method

Conflicts:
	src/main/java/seprini/controllers/AircraftController.java
	src/main/java/seprini/controllers/SidebarController.java
	src/main/java/seprini/models/Aircraft.java
	src/main/java/seprini/screens/EndScreen.java