Implement saving and loading

[numpad]

The player should be able to continue playing exactly where they left, this requires the whole World and everything in it (and possibly much out of the Worlds scope) to be completely serializable.

• All components
• All systems
• All events (?) [1]
• Terrain
• Camera (?) [2]
• AssetManager (?) [3]
• All entities (the registry)
• Settings (Controls, Graphics, Audio)

[1]: Does this make sense? The only case I can currently think of is enqueued events that are buffered until next frame.

[2]: This probably makes sense. Maybe for saving while in a cutscene or something similar.

[3]: Definetly makes sense, but maybe not part of AssetManager? To load all previously active Assets. Maybe this will be handled differently.