Unify plugin code

[Jeremi360]

Rakugo use class_name key word instead of adding nodes and resources inside plugin.gd using add_custom_type().
It was easier this way, but now I think this was a bad approach, as we don't have control on how it loads.
And also make that we can't use features as InspectorEditorPlugin.
So I think we should change it to be inside a plugin.gd using add_custom_type().

[Jeremi360]

This is still open by mistake, we can use InspectorEditorPlugin with class_name