Preserve user variables on current items, allowing for later renaming+metadata updates

[brokkr]

Without Pickle we no longer have access to the entirety of an entry. We there have to think about what data is saved along with the file, if we wish to manipulate it later. Unlike first assumed (see issue #139) we do need more than just a path.

If we save all the same variables that we have access to at download, current items become the equals of new ones. That means that we can perform all the same operations on those, in the exact same way, using the same ressources. That opens up for a neat new feature, where a user with a large back catalogue can rearrange it - i.e. rename or add data to metadata (see issue #110) - without having to redownload or even rely on the entries still being in the feed. Make changes to the config file, run poca, and have it instantly update your collection to match your new preferences.

One area I can think of using this, would be the kind of file that turn out to have terrible file names or metadata. Rather than doing a reset, I can just apply a new name template, drop some new elements into metadata, and update it.