Fix vulnerability to path traversal attacks

Ark was vulnerable to directory traversal attacks because of
missing validation of file paths in the archive.

More details about this attack are available at:
https://github.com/snyk/zip-slip-vulnerability

Job::onEntry() is the only place where we can safely check the path of
every entry in the archive. There shouldn't be a valid reason
to have a "../" in an archive path, so we can just play safe and abort
the LoadJob if we detect such an entry. This makes impossibile to
extract this kind of malicious archives and perform the attack.

Thanks to Albert Astals Cid for suggesting to use QDir::cleanPath()
so that we can still allow loading of legitimate archives that
contain "../" in their paths but still resolve inside the extraction folder.