Refactor Node Processing

* Node processing works on the concept of process groups.
* A node group can be inherited, run on main thread, or a sub-thread.
* Groups can be ordered.
* Process priority is now present for physics.

This is the first steps towards implementing https://github.com/godotengine/godot-proposals/issues/6424.
No threading or thread guards exist yet in most of the scene code other than Node. That will have to be added later.

editor/editor_node.cpp

@@ -636,6 +636,8 @@ void EditorNode::_notification(int p_what) {
 		} break;
 
 		case NOTIFICATION_ENTER_TREE: {
+			get_tree()->set_disable_node_threading(true); // No node threading while running editor.
+
 			Engine::get_singleton()->set_editor_hint(true);
 
 			Window *window = get_window();