Simplify Pingus class layout

[David Philippi]

While trying to work on some bugs, I've noticed that it's very hard to gain 
an understanding of the control flow without asking Ingo. ;-)
The doxygen documentation isn't very helpful since many of the class diagrams 
are much too blown up to read them.

I'd like to start by turning World into a singleton since I perceive it to be 
one conceptually anyway and I don't see any disadvantages in doing so. Any 
objections?

Bye David