Introduce livery-sets on squadron level

Randomly chooses one of the liveries defined if present in the squadron's yaml, overrides entry for "livery" or whatever is chosen in the UI except liveries on flight-member level

game/squadrons/squadron.py

@@ -40,6 +40,7 @@ class Squadron:
     aircraft: AircraftType
     max_size: int
     livery: Optional[str]
+    livery_set: list[str]  # will override livery if not empty
     primary_task: FlightType
     auto_assignable_mission_types: set[FlightType]
     radio_presets: dict[Union[str, int], list[RadioFrequency]]
@@ -503,6 +504,7 @@ class Squadron:
             squadron_def.aircraft,
             max_size,
             squadron_def.livery,
+            squadron_def.livery_set,
             primary_task,
             squadron_def.auto_assignable_mission_types,
             squadron_def.radio_presets,

game/squadrons/squadrondef.py

@@ -27,6 +27,7 @@ class SquadronDef:
     role: str
     aircraft: AircraftType
     livery: Optional[str]
+    livery_set: list[str]
     auto_assignable_mission_types: set[FlightType]
     radio_presets: dict[Union[str, int], list[RadioFrequency]]
     operating_bases: OperatingBases
@@ -103,6 +104,7 @@ class SquadronDef:
             role=data["role"],
             aircraft=unit_type,
             livery=data.get("livery"),
+            livery_set=data.get("livery_set", []),
             auto_assignable_mission_types=set(unit_type.iter_task_capabilities()),
             radio_presets=radio_presets,
             operating_bases=OperatingBases.from_yaml(unit_type, data.get("bases", {})),