Replace setExpanded(const QSet<KUrl>&) by expandParentItems(const KUrl&)

Replace setExpanded(const QSet<KUrl>&) by expandParentItems(const KUrl&)

The use case of this function (Folders Panel) requires the expansion of
the parent items of a single URL, so it's not needed to handle a full
set of URLs in this function. Moreover, the issue that not only the
parents, but also the URLs themselves were expanded is fixed by this
commit.
(cherry picked from commit 89082ca391807abdc26d8985efe6b4c27183a9b1)