Avoid outer folder when decrypting an archive containing a single folder

Kleopatra decrypts an archive into a temporary folder named like the
archive (without suffix). When moving the extracted files and folders to
the destination these temporary folders are moved. This makes sense if
the user decrypted multiple archives with a single operation. In the
most common case that a user decrypts a single archive containing a
single folder (which is the result of encrypting a folder) this resulted
in a folder named like the archive containing a folder named like the
encrypted folder. This unnecessary nesting is now omitted if a single
archive containing a single folder is decrypted. In this case the
destination folder will now contain directly the folder that was
originally encrypted (if necessary with a numbered suffix to avoid
name collisions).

(cherry picked from commit 44a0d67d172d3c68ff408bcffa0af89900189bce)

• GnuPG-bug-id: T8022