Provide customizable fallbacks for SEO fields

It would be great if the fallback title/description/image of a SEO field could be customizable. Right now:

  • the single-line text field marked as the title for the Model is used as SEO title fallback (+ the global SEO title suffix field)
  • the first image present in an image/gallery field is used as SEO image fallback (if missing it uses the global SEO image)
  • SEO description has no fallbacks in the record (it uses the global SEO fallback)