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.
I'm not sold on passing the rallypoint into the unload activity like this.  An alternative approach (which has its own problems) could be to queue a `WaitFor` activity, and then have this trait manage the unloading.  You could then reference an `AttackMove` parameter which can be added to `ProductionInfo` to partially address @penev92's concerns above.