Core API wording: rename CompValue pt.1 (#702).

BREAKING CHANGES

Classes/Interfaces:
{{{
CompositeValue -> ChildTopics
    getCompositeValue() -> getChildTopics()
CompositeValueModel -> ChildTopicsModel
    getCompositeValueModel() -> getChildTopicsModel()
}}}

DeepaMehtaObject:
{{{
getCompositeValue() -> getChildTopics()
setCompositeValue() -> setChildTopics()
}}}

DeepaMehtaObjectModel:
{{{
getCompositeValueModel() -> getChildTopicsModel()
setCompositeValue() -> setChildTopicsModel()
}}}

See #702.
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.
Core API wording: rename CompValue pt.3 (#702).

BREAKING CHANGE

REST APIs: the `fetch_composite` query parameter is renamed to `include_childs`.

Example:
{{{
GET /core/topic/1234?include_childs=true    ### "include_childs" was "fetch_composite"
}}}

Plugin developers are supposed to carefully update client-side code.

NOTE: the former `fetch_relating_composite` query parameter to include the (relating) association's child topics in the response is not supported at the moment. Do we actually need it in the REST APIs? If so, it should be renamed to `include_assoc_childs` and be implemented in the Core's `JerseyResponseFilter` (analogous to `include_childs`).

See #702.
Core API wording: rename CompValue pt.4 (#702).

Just rename variables and adapt comments.

See #702.
Merge branch 'compvalue-wording'