layer control for more prototypes

[hgschmie]

- Loaders offer structure_render_layer to determine on which layer they are rendered and circuit_connector_layer for their circuit connectors
- Crafting machines / Agricultural tower and Mining drills offer circuit_connector_layer in their graphics set
- many more entities also offer a render_layer prototype (e.g. SimpleEntity / SimpleEntityWithOwner)

However, this is far from generalized. E.g. inserters offer neither. It would be great if the entity render layer (and maybe also the circuit connector render layer) would be available at the EntityPrototype level to control the render layers for any entity.