Core: extend DeepaMehtaObject API (#522, #505).

The DeepaMehtaObject base interface specifies a new method:
{{{
void loadChildTopics(String childTypeUri)
}}}
It loads the object's childs specified by its type URI.
If the childs are loaded already nothing is performed.
Can be used for both, "one" and "many"-valued childs.

This method is of interest for Viewmodel Customizer implementers (see #505).

See #522.