Weak reference subtype PointerProperty

As it stands when a PointerProperty is used to point to an existing datablock it is counted as a user, which can prevent the deletion of a datablock when it's not actually used in the scene.

There are times when a property needs to reference a datablock only if it's present and not affect it's actual lifetime. This can be achieved by making a subtype of PointerProperty that acts as a weak reference: only aware of a datablock's existence, not controlling it.