Proper initialization for godot-cpp classes with memnew.
Extension classes (i.e. the `GDCLASS` macro) behave differently from
regular wrapped classes, and requires Godot to initialize them during
object construction.
This commit update the GDCLASS macro to not create/destroy the instance
during the bindings callback, but during the extension callbacks.
When setting the object instance, the bindings instance is set to the
pointer of the extension instance so that it can later be retrieved
normally via `object_get_instance_bindings`.
This makes sure custom constructors are always called on extension
classes. However, note that constructors should not take any parameters,
since Godot doesn't support that. Parameters are ignore in memnew macro.
Use memnew(MyClass()) instead of memnew(MyClass) since it now needs a
value instead of a class name. This macro calls MyClass::_new() (define
in GDCLASS macro) which ultimately calls Godot to create the object,
ensuring that both the Godot and the extension instances are created.
Non Godot classes (that don't derive godot::Object) are constructed as
usual an can have parameters.
memdelete is also changed for the same reason, as it needs to destroy
the Godot object as well, and that automatically frees the bound
extension instance.
