details: Use different icons for repo packages and addons

This makes use of the new x-package-repository and application-x-addon
icons.

https://bugzilla.gnome.org/show_bug.cgi?id=724669
details: Update software source message wording

... as suggested in the ticket.

https://bugzilla.gnome.org/show_bug.cgi?id=724669
Updated Polish translation