Game saveable!

As suggested by Tomcat in godotforums.org, I've implemented a loading/saving system for "Where is Kate?". Doors and drawers status won't be included into the save because I had the problem to reposition every 3D model in the scene with the right orientation.

In some situations I had done some hacking, so I had to completely replaying it more than twice (without cheats) to be sure that the deserializer correctly restore the environment.

The game file is called "game.saved" and is approx. 50 KB. Inside of it there are 188 object status (excluding for doors and drawers).