[API] Broadcast slot change events

[TheIllusiveC4]

Mods that may implement separate GUIs containing Curio slots have a difficult, possibly impossible, task of trying to react to slot changes that occur while the GUI is open. Curios handles this directly by resetting the slots when the packet is received, but this is not viable for other mods that want to do the same thing. The solution is to create a new event for slot changes and broadcast them for mods to handle directly themselves.