Add ability to clone entities in authoring mode

It should be easy to clone entities. This will be useful for placing lots of similiar entities, such as when building a fence or a wall using the stone_palisade models.
The cloned entity should have all of the original entity's attributes. This won't work with entities that have scripts or knowledge attached to them though (perhaps after the advanced entity creator is implemented we can reuse stuff from there though?).
Child entities should also be cloned (though that functionality should be possible to disable).