Intermediate commit: Provide better feedback for bonus payouts
Made the Bonus payout more configurable and routed it through Variation
Modified Variation.getPayout to return -1 if the player lost -> that saves us from calling that method twice in inMotion().
Deregister stuff belongs into hide(), not dispose().
Fixed intendation
Made readyScreen() an abstract method to drive the point home.
Made BureauScreen a bit more abstracter by phasing the stage part out into the new StageScreen subclass -> Bureauengine should no longer be scene2d specific.