Add kio-extras

kio-extras is need e.g. to show thumbs within the KFileDialog (as reported in this issue for Kdenlive). Theoretically it can be added to the apps individually but as the file dialog is probably used by many apps to me it makes sense to add it to the runtime.

However due to the fact that Flatpak mounts files as if created on January 1st 1970 services like ThumbCreator will only be recognized if you delete the cache ($HOME/.var/app/org.kde.kdenlive/cache/ksycoca5_*). That means that atm it only works for users with a new setups, but there is already a patch for KService to solve this problem and as soon as this patch will be applied it will work for everybody. For more info on the patch see also https://mail.kde.org/pipermail/kde-frameworks-devel/2021-June/117900.html