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/NodePath.hpp

@@ -10,6 +10,11 @@ namespace godot {
 class NodePath {
 	godot_node_path _node_path;
 
+	friend class Variant;
+	inline explicit NodePath(godot_node_path node_path) {
+		_node_path = node_path;
+	}
+
 public:
 	NodePath();