Remove redundant colors and method to get colors

See merge request !3

The colors can be get via a method. Necessary for testing.

Fixed undefined ordering of the Turtle color array to conform to docs of setzeFarbe(int)

See merge request !1

# Conflicts:
#   Turtle.java

Refactor colors

See merge request !2

The order of colors being added to the map now matches the order
of the keys in the map. This was necessary because students would
get confused when the order given by the .values() method would
differ form the addition order.

Files generated by BlueJ should be ignored bny git.