TODO: Right now, it's just controlling, the sum of arrows of one type.
You can add max. 10 Arrows per kind.
new methods for dealing with arrows. ArrowIndex <--> ArrowName and
ArrowIndex / ArrowName --> Arrow.class
Mechnics.arrowNumberFreeSetUsable: number of arrows, which can be added
FIXME: look to Spawn-method
Screens improved
MouseListener