Add support for fieldsets

[almet]

When working on v1, we made the decision to remove the support for fieldset, but it's indeed a good feature.

In order to integrate it properly, we would need to:

• Decide on the UX we want to have for the fieldsets: do we want to have them editable or not?
• If we decide to have them editable (I believe it would be a good thing), we then need to have this behaviour working properly: an edited field should remain edited and not be replaced by the default value.