Make Shelves Modular

Request from Discord (Venivici): "Really cool stuff! I think a good idea would be to make all empty shelving units that have multiple shelves for the stamp would be to actually have those shelves each exist on a different sub-layer so we can put things on each sub-layer of the shelf ourselves. Let’s take the empty corner L shaped shelf for example. Maybe when that’s placed the bottom shelf sits on sub-layer 0, middle on sub-layer 1, and top on sub-layer 2 and when we move them up or down they all change accordingly?"