Freeciv

Each 'new' Gtk client (Gtk3, Gtk3.22) migrates settings only from the next older one. So, Gtk3 migrates from Gtk2 (migrate_options_from_gtk2()), and Gtk3.22 migrates from Gtk3 (migrate_options_from_gtk3()).

If a user goes straight from Gtk2 to Gtk3.22 without ever running Gtk3 (quite likely given that we now recommend 3.22; e.g. Debian uses it by default), I think they will not get any settings migration at all; they will go back to the defaults.

Really I think the Gtk clients ought to go back through all previous Gtk clients on the same version looking for settings, with the effective search path looking like:

 :
2.0<--3.0                 .freeciv-client-rc-2.5
       ^
 +-----+
 |
2.0<--3.0<--3.22          .freeciv/freeciv-client-rc-2.6
             ^
 +-----------+
 |
2.0<--3.0<--3.22          .freeciv/freeciv-client-rc-3.0
             ^
       +-----+
       |
      3.0<--3.22<--4.0    .freeciv/freeciv-client-rc-3.1

(This probably isn't as complex as it looks, since we already have the logic to go between versions.)

It's going to be a bit annoying to implement because the Gtk2->3 migration code currently lives in gui-gtk3/ but Gtk3.22 will want a copy.