Feature/30 savegame saving v1

[JaXt0r]

❗ HINT: Can be reviewed and merged after #169

To test

• Save and load the game in multiple ways during a session
  • Is hero spawning correct?
  • NPCs doing their job?
  • Are Items where we left them before saving+loading?

Out-of-Scope/Known issues

• SaveGame menu is always active. So you could potentially hit Save before entering a game and crash it. ;-)
• Thumbnail of an UnZENity save contains the MainMenu. Something to ignore when creating a screenshot later.

[JaXt0r]

I need this branch for future implementations. I tested it a few times during development. As it's an add -on feature, it's safe to accept.