Mission Editor: Refueling Event Handlers should also return the tanker

[Kocrachon]

S_EVENT_REFUELING_STOP and S_EVENT_REFUELING consider the unit receiving fuel as the initiator, but it would be nice if we could also get the aircraft that is refueling them. With missions where you can have more than one tanker, it would be super great to create special events pending on which tanker the aircraft decided to use.