Change the default ordering of ListElementChildren to position

ListElements, ListElementParents and ListElementChildren currently all default to order=name, but in the frontend, ListElementChildren defaults to position and everything else uses name. This makes sense, since the whole point of position is to display elements following the ElementPath.ordering. But since the API and the frontend are using different defaults, this means that API users might be expecting the position order by default.

A quick search on code.vpn shows that exactly nobody uses order='position' in the API client, which means anyone expecting the position ordering is actually getting a name without knowing it. This includes all ML workers, as base-worker does not allow setting an ordering.