Refactor game state

[manuq]

The game state script became big-enough that the gdlint started complaining about the amount of public methods. But more importantly, it is being used for multiple states, namely:

• State that should be persisted to continue the game in a new play session, after exiting the current session. For example: the completed quests, the current scene.
• State that is transient the current play session.
• State that is transient to the current scene. For example: if the intro dialogue has been shown. If the lights are on or off.

This would welcome a refactor.

[manuq]

Marking as blocked because this is something to do later.