POTFILES: Files update

I notice that the POT file currently generated is quite empty, having only strings from desktop and appdata. So I ran extraction of strings for all the files and found the list provided in this MR.

See the resulting connections.pot.

I'm not sure about topbar.ui though. I was not able to find most of its strings in the UI.
(found it: they are actually used in the accessible-name property by AtkObject.)