Research children handling

[bdunogier]

In addition to the Query Field Type (alternative name: dynamic relations, it may be valuable to look into a standardized handling of children.

It could be as simple as a _children_ property on all container items that list DomainContent` items that are subitems of the content (which location ? all of them ?)

The children type can be tested with ... on ArticleType, as a typical container won't contain dozens of items type (it would indicate a bad modelling of the repository).