Add an optional offset property

When entities are placed on the ground they are snapped to it. However, sometimes you want the entity to be adjusted a little, perhaps for making it line up with something else, or for things that are partially buried. For this an optional "offset" property would be useful. It would effectively offset the entity's position in relation to how it would be snapped to the ground (or snapped to anything else).
The property should be expressed as a WFMath::Vector<3> instance.