update dialog: Show full version for updates

When the user clicks around for details, they are likely to be
interested in the exact version, so let's show that.

This brings the update dialog closer to the mockup in
https://bugzilla.gnome.org/show_bug.cgi?id=727981
Show a placeholder when no update details are available

Or rather, fix the existing one to properly show up and update its
wording.

https://bugzilla.gnome.org/show_bug.cgi?id=726501
Updated Galician translations