Create API for saving map/user info

[MartinSGill]

To allow various features that require data to be persisted (e.g. custom POIs etc) a mechanism is required to save that data so that it an be loaded the next time the game starts.

Things to keep in mind:

• How to handle multiple different players? Maybe store data on a per-save basis?
• Where to put the data? Is it safe to use the use save-game folder?