libarchive plugin:
- Make extraction work (was creating empty file).
- Implement extraction of file without preserving the path.
This makes preview work (extraction to a temp dir).
- Get the info whether an entry of the archive is a directory (instead of relying on the name ending with a '/' in ArchiveModel::slotNewEntry()).

So now read libarchive-2.4.11.tar.gz properly :)