Fix Designer scaffolding when inheriting from Collection

Steps to reproduce:

• In Modeler, create a doctype named extendsCollection that extends the default Collection doctype.
• In Designer, scaffold the view layout for extendsCollection. It would be expected to scaffold a layout that is based on the default view layout for Collection, but instead you get layout without any elements.

At first, it seems that we're failing to parse the elements of the Collection view layout (here).

This issue was discovered by Fábio while validating this QA ticket.