Add a way to have "fallback" items

This is especially useful in translations. If you have incomplete translations for an item, you want it to fallback the empty fields to the "next" translation in line, for example:

Dutch-België
title: Hallo wereld!
description: _empty_

Dutch-Nederland
title: Hallo daar!
description: Welkom op mijn website

should result in

title: Hallo wereld!
description: Welkom op mijn website.

This could be potentially be done with a fallback query parameter that will merge all nested o2m items into a single object, where nulls fallback to the next in line:



with the param it could become