Simplify relations to nested entries

[thupsi]

As detailed in #13794 (comment) it is possilbe to add nested entries in an entries field, but only in a roundabout way.

Additionaly, according to #17703 (comment), if you don't create a custom source and opt to select "All" sections in an entries field the entries are "pre-filtered" and nested entries don't show up. I assume that's similar to quering entries with .section('*').

I think it would be great if there was an additional option in the entries field, something like Allow nested entries.

A nice side-effect of this feature is that it would bring matrix fields closer to top-level sections in terms of functionality, which, in turn, helps with the "relating an entry to a section" problem.

Hope that makes sense 🙂