I only translated the undo menu into GtkBuilder's .ui format for now.
The only missing part is that the icon is now shown.
Note that in various parts, I don't rely anymore on a bogus menu action (i.e.
"undo-popup" action in this case) which does nothing but has an associated label
and icon. I simply add the label and icon as submenu attribute directly in the
.ui file, which is translatable and whose strings should be parsed by gettext.
Eventually I'll just get rid of all the various "*-popup" or "*-menu" bogus
actions.
