Refactor the code

[w0rm]

This time it increases the code, but I made some changes that make things nicer:

• moved entities into the Bodies module
• moves sizes into the Bodies module
• renamed the custom types
• collapsed the ball in hand with the ball behind the head string — this has simplified the code quite a bit!
• added comments to a few places in the code
• removed cueBallPosition from the playingState in favour of reading it from model.world
• renamed playingState to cue