Support script global class (class_name) when importing a scene

We could already choose a script global class for root_type at scene import config. However, it would fall back to default Spatial if a script global class is chosen. This will make sure the base type for the script class is used, and the script to root node is attached upon import.

editor/import/resource_importer_scene.cpp

@@ -1239,6 +1239,13 @@ Error ResourceImporterScene::import(const String &p_source_file, const String &p
 	}
 
 	String root_type = p_options["nodes/root_type"];
+	root_type = root_type.split(" ")[0]; // full root_type is "ClassName (filename.gd)" for a script global class.
+
+	Ref<Script> root_script = NULL;
+	if (ScriptServer::is_global_class(root_type)) {
+		root_script = ResourceLoader::load(ScriptServer::get_global_class_path(root_type));
+		root_type = ScriptServer::get_global_class_base(root_type);
+	}
 
 	if (scene->get_class() != root_type) {
 		Node *base_node = Object::cast_to<Node>(ClassDB::instance(root_type));
@@ -1251,6 +1258,10 @@ Error ResourceImporterScene::import(const String &p_source_file, const String &p
 		}
 	}
 
+	if (root_script.is_valid()) {
+		scene->set_script(Variant(root_script));
+	}
+
 	if (Object::cast_to<Spatial>(scene)) {
 		float root_scale = p_options["nodes/root_scale"];
 		Object::cast_to<Spatial>(scene)->scale(Vector3(root_scale, root_scale, root_scale));