Import using Download Client API

Squash and rebased only those parts of the torrent collaboration that facilitates the  DownloadTrackingService with the new import logic.
The UI changes work toward torrent, by adding a Usenet header to the Thingy Add UI.

See #76 for more details.

Some known issues:
* Need to add the UI to be able to select the predefined urls, mostly for newznab.