Better Airdrop Production

This change allows the Production class and its children to take in multiple actors to be produced.
This is needed for #7055 so that the units produced can exit in an orderly fashion.
That's a fair point. I'm not excited about actors exiting with AttackMove actually. Weirdness aside, the Starport usecase will have issues with this.
Imagine an enemy in your base, close to your starport. You ordered 5 tanks. The first two get off the space frigate and stop to shoot because they are AttackMoving. The next 3 won't be able to come out until one of the first two moves.