Use macros to enforce "hidden object" constraints, and rename them to "hidden events"

[Rangi42]

See also #557. Since we're calling those "toggleable objects" instead of "hide/show objects", there's no potential confusion in also calling these "hidden". However, these are not objects; there's no relation to object_events. Instead, they are more like bg_events or other map events, just not declared in the same per-map files.

[dannye]

Technically, this is just the event function's argument, generally speaking.

[dannye]

This is pretty much redundant with:

HiddenEventPointers:
; each of these pointers is for the corresponding map in HiddenEventMaps
FOR n, num_hidden_event_maps
	dw HiddenEventsFor_{HIDDENEVENTMAP{n}}
ENDR

since the common mistake would be to forget to add a hidden_events_for / hidden_event list for some map, for example by removing:

	hidden_events_for ROUTE_4
	hidden_event 40,  3, GREAT_BALL, HiddenItems
	db -1 ; end

but then the error you'll get is Undefined symbol `HiddenEventsFor_ROUTE_4` inside of the previous dw for-loop. I think the only way to get this `hidden_events_for ROUTE_4` is not defined error message is to meddle with the previous for-loop, which a user is unlikely to do.

I'm not explicitly asking for it to be removed, but I think its value is minimal (even though its error message is nicer than the one that is superseding it).