Display the position of new high scores
Simplify logic around removing bullets
Be better at closing created graphics objects
Use Graphics2D over Graphics wherever possible
Further uses of dispose() for Graphics2D objects
Use a better .distance() method
Fix occasional NullPointerException from JumperTower
Fix possible NullPointerException
Remove handling of a NullPointerException that should never happen
Remove bulletsToAdd hack from AbstractTower

JumperTower was the only implementation that used it, and it can do its
jumps in a different way.

Also remove the jumpRangeDividend, and make the jump range equal to the
tower's ordinary range, for simplicity.
Fix spelling mistake, earnt -> earned
Remove Helper.toPoint(), by using Point2D where appropriate