Unhelpful listing in software-properties/-sources

For Canonical Partners, we should use "Canonical Partners", and for purchases we should use something like "Purchased software". I don't know what we could do for other sources, though. And in a future version I expect we'll be able to add arbitrary sources directly from USC rather than going into Software Sources, so this bug will become steadily more important. Ideally we'd use the Label of the archive, but I guess there's a catch-22 there in that we don't know the Label until after the repository data has loaded, which is after we've already added the line to sources.list.

@mpt: We can query the repository data for PPAs without having to add them to the sources, as it's done in apt-add-repository to confirm that you want to add the repository, so we could query the name/short description. However there still are some blanks, like medibuntu or other (non-PPA) repositories used, for which we either have to add them to the software sources, or build the repository description file URL and read the description from there (From the release file we can get the name of the release, and we should map components like main, universe, etc to human-readable names).