Core API wording: rename CompValue pt.2 (#702).

BREAKING CHANGE

The topic/association's JSON property `composite` is renamed to `childs`.
{{{
{
    "id": ..,
    "uri": ..,
    "type_uri": ..,
    "value": ..,
    "childs": ..    ### was "composite"
}
}}}

Plugin developers are supposed to carefully update the JSON migrations and client-side code.

See #702.