Don't add recursively every file copied to desktop

When a folder is droppen on the desktop, every file contained in the
folder and subfolder was added to the screen mapping, polluting the
entry a lot

this because KIO::CopyJob::copyingDone is emitted for every file that's
copied, not only the single folder

Add an update script that forgets all urls in screenmapping that begin with
desktop:/ but are it its subfolders rather than files directly in desktop