Implement G1 savegame loading - Items + Hero position

[JaXt0r]

Our game only supports New Game at the moment. We want to support official G1 savegames. As ZenKit supports save&reload, let's leverage it to have G1 compatible save games for Gothic-UnZENity.

Links:

• SaveGame.cs at ZenKit
• TestSaveGame.cs at ZenKit.Tests

TODOs

• [x] Create a Gothic save slot where you changed some stuff. (e.g. moved item, killed NPC)
• [x] Implement test to load this save game
• [x] Check if the loaded assets match expectations and if the world is correctly loaded
• [x] Implement Loading menu inside main menu (according to G1 look&feel)