Move unit delivery ownership out of the UI.

Breaks save compat, but we need to have this knowledge outside the UI so
we can know whether or not we can ferry aircraft to the airfield without
overflowing parking.

game/theater/controlpoint.py

@@ -33,6 +33,7 @@ from .theatergroundobject import (
 if TYPE_CHECKING:
     from game import Game
     from gen.flights.flight import FlightType
+    from ..event import UnitsDeliveryEvent
 
 
 class ControlPointType(Enum):
@@ -158,6 +159,7 @@ class ControlPoint(MissionTarget):
         self.cptype = cptype
         self.stances: Dict[int, CombatStance] = {}
         self.airport = None
+        self.pending_unit_deliveries: Optional[UnitsDeliveryEvent] = None
 
     @property
     def ground_objects(self) -> List[TheaterGroundObject]:
@@ -387,3 +389,7 @@ class OffMapSpawn(ControlPoint):
 
     def mission_types(self, for_player: bool) -> Iterator[FlightType]:
         yield from []
+
+    @property
+    def available_aircraft_slots(self) -> int:
+        return 1000