Fix container and string leaks

Some functions return a new instance of such containers,
but instead we made a copy of them, without taking ownership of the
original created by the function.
Now we use a specific constructor taking ownership on the godot_* struct.

include/core/Wrapped.hpp

@@ -5,6 +5,7 @@
 
 namespace godot {
 
+// This is an internal base class used by the bindings. You should not need to access its members.
 class _Wrapped {
 public:
 	godot_object *_owner;