Make folderview understand application/x-kde-dndextract.

The code was copied almost as-is from Dolphin's implementation. It
should now be possible to drag and drop files from any archive opened
in Ark to folderview.

It previously worked with some archive types due to a weird code path
in Ark which should not exist.

PS: Fredrik, I ended up using pathOrUrl() because I can't pass KUrl
via DBus.

Reviewed-by: Fredrik Höglund