Feat: Nested collections

[johannesschaffer]

This is especially useful with my previous feature request, Repeatable & Non-repeatble zone in a collection. Otherwise this functionality could somewhat be achieved with groups. But combined with the zones it would enable something like:

• Topics
  • [Non-repeatbale]
    • H1
    • Description: What we de in out topics
  • [Repeatable]
    • Topic 1
      • [Non-repeatbale]
        • H1
        • H2
        • Description: What is this topic about
      • [Repeatable]
        • Article 1
        • Article 2
    • Topic 2
    • ....
    • Topic 3
    • ......