Use KIO::moveAs to move decrypted folders to the target folder

Our own helper didn't move hidden files and failed when trying to delete
a directory containing a hidden file. Moreover, using a KIO job we
get a progress dialog for free.
Use KIO::moveAs instead of KIO::move to avoid the source folder being
moved into an already existing target folder. This shouldn't happen
because we explicitly try to find a non-existing name for the target
folder, but better safe than sorry.

GnuPG-bug-id: 6373