Scaffold file:content property while generating layouts from File subtypes

Currently, when creating a File subtype in Studio Modeler, and then generate the associated layouts, the file:content property is not scaffolded, which really breaks the user experience (as you have to manually add it). Goal is to have it automatically, with a the same element for file:content as the File document type.

The reason of the existing behaviour is because no fields coming from “default” schemas are ever scaffolded automatically (to avoid having to scaffold all the fields from a Document Type).