First working cut of magnets

This iteraction is currently dependent on dn inclusion in the magnet.

It would be better to use the namespace from the rss feed for both
magnets in files. This requires significant changes to the feedchecker
and downloader, and migration from older preferences formats.
Storing Title in the Downloaded history

Storing title in the download history enables the recently downloaded
list to show the real name. It also provides a reliable name for
magnets.

To Do: Handle migration of old preferences file formats.
Migration of downloads history to new format

Change old array of strings to new dictionary format when detected.