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/campaignloader/squadrondefgenerator.py

@@ -47,6 +47,7 @@ class SquadronDefGenerator:
             role="Flying Squadron",
             aircraft=aircraft,
             livery=None,
+            livery_set=[],
             auto_assignable_mission_types=set(aircraft.iter_task_capabilities()),
             radio_presets={},
             operating_bases=OperatingBases.default_for_aircraft(aircraft),