libarchive: Stop manually handling temporary work files.

Having to call QFile::remove() whenever there is a failure and we want
to get rid of the working file is bad -- for example, we were not
calling it when archive_write_header() failed in deleteFiles() and thus
could end up with a potentially big dangling file.

By using QSaveFile, we solve that problem and also stop using
predictable names for the temporary files we work on when adding and
deleting entries.

Based on commits de3cc8dc and 972f1016 from master branch, but modified to use QSaveFile instead of KSaveFile.