Circular References between PBCPlay and PBCCategory

[obraunsdorf]

Every play has a reference to its categories and every category has a reference to its plays.
This yields circular references which could lead to memory leaks.
More important, for every new feature we have to carefully make sure that every reference is kept up-to-date. This makes it a little more difficult to implement new features.
We should consider to make this relationship unidirectional.