AircraftController: initialise

[CremboC]

Generate permanent waypoints. Choose from (list of?) hard-coded options.

[CremboC]

@PWDrew @josephhamm you now have the game difficulty in the constructor of the class.
GameDifficulty.java:

public enum GameDifficulty {
    EASY, MEDIUM, HARD
}

In the constructor of AircraftController you now have:

public AircraftController(GameDifficulty diff, Stage stage, InputHandler input) {
    // ...
}

You can do something like

if (diff == GameDifficulty.EASY) {
    // do something for easy
}

if (diff == GameDifficulty.MEDIUM) {
    // do something for easy
}