getEventHolder
Searches this holder's own subtree (this event, then depth-first through events and tiles) for the EventHolder whose id equals eventId — the lookup behind TriggerEvent/UpdateEvents, when it recurses past the point where TilesManager.getEventHolder handed off into a specific event's own nested structure.
Return
the matching EventHolder, or null if not found in this subtree.
Parameters
eventId
the id to search for.