Allow Z-Kill Modification on World Settings

With the introduction of Star Wars and “space” if you create an island, that need vertical “freedom” if you forgot to change or consider to start the map with a good offset for it, you get killed when flying under the Z Kill value. And the “Fix” of moving everything up when there are thousand of assets in the level is really tedious and slow. Specially if you try a bulk operation, it can stall some machines.