Make extension instances create the corresponding godot object in their constructor

[groud]

This requires godotengine/godot#55470 to work.

First, the Godot object creation is the first step executed when creating an extension instance. This is done by making the default constructor call the parent one in the initialization list. The topmost Wrapped constructor thus call the Godot object creation as soon as possible (using classdb_construct_object).

As we want to keep the possibility to create a default Constructor in a custom class (those using the GDCLASS macro), I could not pass the final p_godot_class parameter to the Wrapped(const char *p_godot_class) constructor with the actual class name. Instead, the name of the first class built-in to Godot is passed. This is thus enough to construct the Godot object, but not to finalize the initialization correctly.

As a consequence, i used the same trick as we have in Godot, by modifying the memnew macro so that it sets the object instance after the object is created. The memnew calls a _postinitialize function, that registers the instance to the object using godot::internal::gdn_interface->object_set_instance(_owner, _get_class(), this);. There, the _get_class function is available, which allow finally setting up the object correctly.
Note: this cannot be done in the Wrapped constructor as, at that point, the virtual functions are not initialized yet. So calling _get_class() in the Wrapped constructor would not compile.

Finally, to avoid users creating object without using memnew, I made operator= private. The same trick is used in godot so that should not be a problem.

[akien-mga]

Thanks!