Multiple circle shapes in a body with position

[Sorry for the closed pr, I forgot to pull in your latest commits] 
See the old comment 'a circle has unfortunately no position in p2. pretty useless.' ? What a lie.
It works. There was only an error in the debug drawer and I did not realized that I can use offsets even for circles. That's nice! I can now place multiple circles in physics editor and they all are placed inside the body at the correct position.