[WIP] Caravan circuit improvments

[mnemonicly]

Hello,

I'm working towards improving the circuit conditions in caravans. Before I get too far, (and because I'm bad at LUA) I wanted to start a PR for feedback.

At the moment, this adds a "circuit condition with static value" option to caravans, to allow checking if "=" a value, rather than another signal. As far as I can tell, the nice circuit gui dialog that the base game has cannot be reused by mods, so this seemed like the cleanest solution, but I might be missing something.

I'd like to also add support for a wider range of conditions, not just "=". I'm not exactly sure the best way to do this. My thought is to make a circuit condition class that encapsulates the conditions and conditionals, and exposes an evaluate method, but I'm not sure exactly where to stick this class, or if theres a simpler way I'm missing.

I'll keep poking at my ideas, but would appreciate some feedback before I go too far down a road.

[notnotmelon]

I love this, really good idea. Code looks clean. Is it ready for merge?

[mnemonicly]

This feature should be mergable. I'm working on a related one to allow for conditionals that aren't "=" but that can be a separate PR.