My assumption that a PreviewJob can remove its temp dir when it's done was
wrong, at least with nested archives. If we remove the extracted file too
soon, there is no way to preview a nested archive or, worse, a "double-nested"
archive.

This patch just delays the cleanup to the Part destructor (= when we are
closing the app).

• Open https://go.skype.com/skypeforlinux-64-alpha.deb
• Preview of data.tar.xz now works.
• Make sure that the extracted file in /tmp is deleted on Ark quit.