Added road demolishing!

Roads can now be demolished! Also, if you are in demolish mode and you hover over a road, it will highlight the road red. Some roads cannot be demolished, only roads that you placed can be demolished.

This feature seems really simple but it was a huge pain to implement, mostly because of the crazy amount of pointers to other objects everywhere. Also, remember how I said I shouldn’t be using size_t for both physical and internal IDs, well that came back to bite me. I spent like an hour debugging that, but at least that’s fixed now!