Let the TheaterCommander manage front line stance.

This improves the AI behavior by choosing the stances non-randomly:

* Breakthrough will be used if the base is expected to be capturable and
  the coalition outnumbers the enemy by 20%.
* Elimination will be used if the coalition has at least as many units
  as the enemy.
* Defensive will be used if the coalition has at least half as many
  units as the enemy.
* Retreat will be used if the coalition is significantly outnumbers.

This also exposes the option to the player.

game/settings.py

@@ -55,6 +55,7 @@ class Settings:
     automate_runway_repair: bool = False
     automate_front_line_reinforcements: bool = False
     automate_aircraft_reinforcements: bool = False
+    automate_front_line_stance: bool = True
     restrict_weapons_by_date: bool = False
     disable_legacy_aewc: bool = True
     disable_legacy_tanker: bool = True