Add support for @@girders-elements/children property

[ognen]

An element can include a property @@girders-elements/children which denotes which of it's properties contain children elements. These properties can either contain an element or an array of elements.

This property can contain:

• a string denoting a single property name, (e.g. '@@girders-elements/children': 'children')
• an array of strings denoting which properties are child elements
  E.g.

{
   kind: ['container', 'two-column'],
  '@@girders-elements/children': ['main', 'aside'],

  main: [
    { 
       kind: 'tile',
       content: [ ],
    },
    // ...
  ],
  aside: {
    kind: 'weather',
    content: [],
  }
}

Support required:

• uiFor with no arguments produces the UI for all child elements
• adjust the element zipper to take into account this property (defaulting back to the one in configuration), after which the zipper can be positioned:
  • on an element
  • on a named collection of element