Support for more advanced stuff like NBT copying wasn't added in this PR, as the NBT used by PM is currently an inconsistent mess and doesn't play nice with vanilla. In the interests of avoiding this mess propagating, it's been left for another time.
Adds PlayerEntityPickEvent a la PlayerBlockPickEvent
and Entity->getPickedItem().
This was static to permit ItemFactory to register spawn eggs for all known entity types in early PM4. However, nowadays we provide a callback to the spawn egg instead, and spawn eggs must be manually implemented, so this is no longer needed.
In addition, having this static forces everyone to make a new entity class for every unique type of entity, which isn't ideal.
