app, plug-ins: mark missing strings as translatable
This marks most strings I could find in the GUI as translatable.
Mostly follow-ups to the GimpProcedureDialog port, but there were some N_
that needed replacing with _
as well to be used.
Saw a handful of strings that were less straightforward, but I'll take them in a later MR, or file a bug report if I can't figure them out.