Right now visited_events_history is stored globally, meaning slot-independent. This is useful for when you want to allow the player to easily skip sections they have read before.

However there are other situations where it can be useful to know if you've visited an event in this run/slot, e.g. choices currently use the visited_events_history to provide a "visited_before" variable to _load_info(), but it's global, which is usually not what you'd want.

So we should add some way to have both independantly of each other.