Further changes in GDNative API

- Added new_copy to all types, since trivial copy won't work for all
  types.
- Added functions to convert from String to char array types, which is
  not provided by the methods bound in Variant.
- Added operator index to String.
- Added missing cstring version of some Variant functions. They existed
  in the header but didn't have the implementation and were missing from
  the gdnative_api.json file.
- Added support for static calls on Variant types.

modules/gdnative/gdnative/node_path.cpp

@@ -42,6 +42,10 @@ void GDAPI godot_node_path_new(godot_node_path *p_self) {
 	memnew_placement(p_self, NodePath);
 }
 
+void GDAPI godot_node_path_new_copy(godot_node_path *r_dest, const godot_node_path *p_src) {
+	memnew_placement(r_dest, NodePath(*(NodePath *)p_src));
+}
+
 void GDAPI godot_node_path_destroy(godot_node_path *p_self) {
 	NodePath *self = (NodePath *)p_self;
 	self->~NodePath();