externals property
The widgets a scene may place, as the app declares them.
The editor sends a scene as DATA, and a generated arguments class is
not data — it cannot travel, and the wire carries the entry label
instead. So the app hands over the same declaration list the generator
read, and the label finds its widget here.
A widget the app forgets to declare shows as its label rather than blank.
Implementation
final List<ExternalWidget> externals;