LDCad Set PBG generator

[Orion Pobursky]

I've made a few "under the hood" changes to the code to allow for better code reuse in other projects and to try to ensure that we don't exceed the request/sec limitation imposed by Rebrickable. If you notice anything odd, as always, report it here. I have a path forward for automatic sticker inclusion but have to think more on how to handle sticker shortcuts.