Refactor the sell unit changes as requested.

It works more simply now, and also doesn't immediately sell the unit.

Also adds a matching UI dialog popup for selling too many ground units.

game/theater/base.py

@@ -29,7 +29,6 @@ class Base:
         self.aa: Dict[AirDefence, int] = {}
         self.commision_points: Dict[Type, float] = {}
         self.strength = 1
-        self.sold_units = {}
 
     @property
     def total_aircraft(self) -> int: