Signal Change event

[scruffyvoltherder]

It would be really useful to have some sort of signal change notification so that a mod doesn't have to use up UPS by constantly polling. I'm thinking that the mod could call a function to register the LuaWireConnectors of entities that you care about. There would then be a global table of connectors where the signals changed last tick. I'm thinking the key could be the LuaWireConnector object, and the value could be the LuaCircuitNetwork object associated with it.