Join some classes together

The claims step is no longer needed since we have no claims table.
Having distinct EntityInsertionStrategies for Items and Properties
is also not needed at present.
I did this to avoid binding to the class names. You can now use a different implementation of item if you want, and stuff will not break if the classes where to be moved. Those are very minor points, and I basically mainly want to get an idea of how this will actually hold up in the future. I don't expect any real difference compared to the instanceof approach.