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.

gen/ground_forces/ai_ground_planner.py

@@ -117,7 +117,6 @@ class GroundPlanner:
                 print(key)
                 continue
 
-            self.cp.base.sold_units[key] = 0
             available = self.cp.base.armor[key]
             while available > 0: