Fix add_root_node() being no-op

doc/classes/EditorScript.xml

@@ -44,8 +44,7 @@
 			<return type="void" />
 			<param index="0" name="node" type="Node" />
 			<description>
-				Adds [param node] as a child of the root node in the editor context.
-				[b]Warning:[/b] The implementation of this method is currently disabled.
+				Makes [param node] root of the currently opened scene. Only works if the scene is empty. If the [param node] is a scene instance, an inheriting scene will be created.
 			</description>
 		</method>
 		<method name="get_editor_interface" qualifiers="const" deprecated="[EditorInterface] is a global singleton and can be accessed directly by its name.">
@@ -57,7 +56,7 @@
 		<method name="get_scene" qualifiers="const">
 			<return type="Node" />
 			<description>
-				Returns the Editor's currently active scene.
+				Returns the edited (current) scene's root [Node]. Equivalent of [method EditorInterface.get_edited_scene_root].
 			</description>
 		</method>
 	</methods>

editor/editor_script.cpp

@@ -32,7 +32,10 @@
 
 #include "editor/editor_interface.h"
 #include "editor/editor_node.h"
+#include "editor/editor_undo_redo_manager.h"
+#include "editor/gui/editor_scene_tabs.h"
 #include "scene/main/node.h"
+#include "scene/resources/packed_scene.h"
 
 void EditorScript::add_root_node(Node *p_node) {
 	if (!EditorNode::get_singleton()) {
@@ -41,11 +44,24 @@ void EditorScript::add_root_node(Node *p_node) {
 	}
 
 	if (EditorNode::get_singleton()->get_edited_scene()) {
-		EditorNode::add_io_error("EditorScript::add_root_node: " + TTR("There is an edited scene already."));
+		EditorNode::add_io_error("EditorScript::add_root_node: " + TTR("The current scene already has a root node."));
 		return;
 	}
 
-	//editor->set_edited_scene(p_node);
+	const String &scene_path = p_node->get_scene_file_path();
+	if (!scene_path.is_empty()) {
+		Ref<PackedScene> scene = ResourceLoader::load(scene_path);
+		if (scene.is_valid()) {
+			memfree(scene->instantiate(PackedScene::GEN_EDIT_STATE_INSTANCE)); // Ensure node cache.
+
+			p_node->set_scene_inherited_state(scene->get_state());
+			p_node->set_scene_file_path(String());
+		}
+	}
+
+	EditorNode::get_singleton()->set_edited_scene(p_node);
+	EditorUndoRedoManager::get_singleton()->set_history_as_unsaved(EditorNode::get_editor_data().get_current_edited_scene_history_id());
+	EditorSceneTabs::get_singleton()->update_scene_tabs();
 }
 
 Node *EditorScript::get_scene() const {