Add type hints to all GDScript files

[Calinou]

The codebase currently has some type hints, but many of them are missing (especially for function return types).

If you'd like, I could go through the codebase and add them. I just need to know whether you're OK with type inference (:=). I personally prefer using it, but I know some people don't 🙂

[RodZill4]

I try to add types when modifying the code, but I started this project before types became available in Godot. I'm a C/C++ developper when I don't use Godot, so I'm kinda okayish with strong typing. ;)

[RodZill4]

I grep'd all missing return types and it seems the remaining ones cannot be fixed. Let's close this now (and be a bit more careful in the future).