Refactor: Game creation

[esbudylin]

This PR refactors the game creation code. It decouples the logic for setting up a new game from PlayerSetup into a new class, GameSetup. The reasoning is to make PlayerSetup responsible only for UI logic, while separating the "model" into the GameSetup.

There are also a few smaller improvements:

• The GlobalSingleton now directly holds the SaveGame instance produced by setting up a game. There is no longer need to dump this data to a temporary JSON file and re-read it after entering the game scene.
• The "barbarian" flag is now a property of a civilization, not a player. It was done to avoid error-prone string look-ups in the PlayerSetup.