Adding load and save game logic plus better menu

Ok, loading now works as well. I think this is as far as I want to take it right now, there are a number of things that we need to do but I think they kind of fall outside of this initial PR.

A UI somewhere in the game, either a wrist UI (my preference) or a popup menu, that gives you access to options such as reload from last save, quit game, etc. This might also be a start to a UI that allows you to adjust in game things.
The current "save pole" only has a big red button you can press that functions as an autosave, saving the game using a name that is generated once after a new game starts and is then reused. We need a second alternative that allows you to give the save game a name but this would require a more elaborate UI. This could be implemented in the wrist UI but I'm a strong believer in having predefined save points in a game instead of being able to save at any moment (though both should be, and are, possible).
I didn't do anything nice with the summary, I think that is definately something we can improve upon.
The menu in the main scene is bare bones, we can probably dress it up even further.

GodotVR / godot-xr-template

Adding load and save game logic plus better menu #17