The use of `App::PropertyLinkSubList` for `'PathSubelements'`
is a mistake because we could use a single `App::PropertyLinkSub`
to handle both `'PathObject'` and `'PathSubelements'` properties.
This commit doesn't change any code, it just adds comments
explaining the situation so that it is not forgotten,
and we remember to address it in the future.
Ideally we should migrate the objects, but we may also decide
to break compatibility with older `PathArrays` if both properties
can't be migrated easily.
327b306307