appDisplay: Only create default folders on the first start

appDisplay: Only create default folders on the first start

If all folders were removed, after a session restart the default folders
would have been recreated again, effectively preventing users from
using the app grid without folders.

Avoid this problem by only creating the default folders if the
folder-children setting has never been modified before.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8713
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4047>