> So, you have to register OpenDoor() with that handle "open_door" (or whatever name you want to have) and there is no way to call it directly? What if two objects define the action "open_door"?

Yes, actions live in a global namespace. That's because most actions for the editor are global anyway (like messages, game goals, create objects, etc.), and they all end up in a big dropdown list for the editor actions. It may make sense to introduce additional "per object" actions. For the editor, all actions must end up in a big dropdown list anyway, so there's no good way around that global list. It may have been a good idea to prefix action names though (e.g. calling them Door.open instead of open_door) to avoid conflicts.

>  and there is no way to call it directly?

You can always call door->Open() (or whatever the function is called). I don't think the trigger system should replace all regular object calls.

> Regarding that "signal" thing

"signal" is fine. It's just that in Qt, "signal" is the trigger. You have signals, slots and connections between the two. I can see how the signal could also be the connecting piece (as in "a signal going over a wire").