Convert structure field types to page collections

I found your snippet for related pages in the [documentation](http://v2.getkirby.com/docs/solutions/blog/related-articles) and was searching for a way to allow users to add a list of pages within the panel. 

Therefor the structure field type looks like a perfect fit. 

Example blueprint definition for a related page structure:
```yaml
related:
    label: Related Posts
    type: structure
    entry: >
      {{title}} ({{uri}})
    fields:
      title:
        label: Title
        type: text
      uri:
        label: Post
        type: page
```

Example for related post entries in the content file:

```yaml
----
Related:
-
  title: Related Post A
  uri: blog/post-a
-
  title: Related Post B
  uri: blog/post-b
----
``` 

Unfortunately the `pages()` function on the field does only convert plain lists of URIs to a collection of page objects. With this addition structures like the described one would be converted as well, as long as `uri` is used as field name.