Ensure const correctness for wrappers

[AThousandShips]

The pointer validity for casting is broken with multiple inheritance and possibly other situations, this ensures that pointers returned match the constness of the relevant functions

I'm assuming these are internal and don't affect compatibility in any way, most users likely aren't using multiple inheritance for interfaces etc. so probably why it has gone unnoticed

• Fixes: Multiple Inheritance breaks when _to_string function is declared. #1232

[dsnopek]

Thanks! Looks good to me and matches how Godot handles similar methods.

[AThousandShips]

Thank you!