Refactored score into it's own class. Broke the aircraft tests and I'm
not sure how to fix them.

Added a method to draw the multiplier level and the background for the
multiplier bar. Still need to make the actual bar.
Merge branch 'ScoreAndMultiplierDisplay' of https://github.com/liam-m/SEPR-MQV-2.git into ScoreAndMultiplierDisplay