Enforce that custom nodes keep their original type (assigned script)

[bjornmp]

I propose that custom nodes registered via class_name, EditorPlugin.add_custom_type() or the [GlobalClass] attribute in C# and added to the scene via the "Add Node" dialog, should permanently retain their original type (registered script). In practice this means:

• Type continuity: It should be impossible for the user to (accidentally) clear the original script of a custom node that was added via the "Add Node" dialog. This guarantees that a custom node will never lose its intended type.

• Extensibility: The user should be able to extend custom nodes as usual (create a script that inherits the original type and replace the original script of that node with his own). However, if he then clears his extension-script from that node later on, the custom node should revert to its original baseline-script instead of becoming a non-scripted native node.