bz19904: fix titles for youtube videos.

Don't reset torrent_title to None for HTTP downloads, we use that for
youtube videos to set the title of the item.

Changed it so that if torrent_title gets changed, then we call
calc_title() automatically.  Before we weren't doing that so the title
stuck around until the download completed.  This fixes #19900
bz19890: handle corner cases of starting playback

Handle the user using File->Open to start playback, but some or all of
the items not being playable.
bz19893: fix AttributeError in migration code