[packagekit] Sorting the update list

[Richard Hughes]

In https://bugzilla.redhat.com/show_bug.cgi?id=436971 it was noted we
don't sort the output list.

Now, where would be a sane place to do this:

* the backend (not, as would limit to sync)?
* the engine when we proxy (would increase CPU load for many Packages)
* the client tool when we display them to the user

I'm erring on the latter.

Ideas welcome.

Richard.