this allows assuming that a position has a valid world in places where it's never expected to not be valid. Since this is the vast majority of usages, it eliminates a lot of possible null-pointer warnings given by static analysers.
TODO: Consider whether we can make Position->getLevel/World use this behaviour out of the box in the next major version.
Inventory holders are now freed from BaseInventory. They are now declared by subclasses by convention, but are not required in most cases.
Ideally, this would be followed by the removal of the need for inventories to know their holders at all. They should just be simple containers of items.
This fixes#1560 by removing FakeBlockMenu.
