godot 4.0
Updated version of #7.
I think the only thing that doesn't work yet is editing the containers from the inspector. I didn't understand how to do that, so maybe you can help me with that too.

I have changed all mistakes, but I can't change in the Layout resource the Root resource. It is set to "Tabs" and not Split like in your 3.x Version. How can I change that?
I have updated it now, but it dosn't save the layout in the editor. (after restarting the editor it shows the correct layout until clicking on the dockable container)
Yeah, I tested here, super weird =S Whenever I select the DockableContainer object, it resets the layout. There is probably something weird going on when the property editor is instantiated / updated. This will take some time to debug.
By the way, apparently EditorProperty.update_property was renamed to _update_property (https://docs.godotengine.org/en/latest/classes/class_editorproperty.html?highlight=editorproperty#class-editorproperty-method-update-property)
I'm closing this pr to create a clean one for Godot 4.0.