Enforce that custom nodes keep their original type

Enforce that custom nodes and resources created via the "Create New Node" dialog, should permanently retain their original type (script). This means:

- Type continuity: It should be impossible for the user to (accidentally) clear the original script of a custom node that was created via the "Create New Node" dialog.

- Extensibility: The user should be able to extend custom types 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 type should revert to its original script instead of becoming a non-scripted type.

scene/scene_string_names.h

@@ -143,6 +143,8 @@ public:
 	StringName shader_overrides_group;
 	StringName shader_overrides_group_active;
 
+	StringName _custom_type_script;
+
 	StringName pressed;
 	StringName id_pressed;
 	StringName toggled;