Fix cyclic reference between Unit and UnitCategory

This made Unit/UnitCategory practically undeletable, as seen for example by the numerous valgrind/LSAN complaints for the unit tests. To solve this the reference to UnitCategory inside Unit is now just a raw pointer.

This is unfortunately not the only problem here, UnitCategory being polymorphic but treated as a value type causes slicing. This works by chance as the sub-classes only contain logic in their ctor, but have no member variables or overloaded methods. This is why this solution works, but the slicing also happened before already.