Plug-in localization was always partially plug-in side, especially for
things like custom GUI. But labels or blurb in GIMP (such as in menus or
action search) were localizing GIMP side.

It had many drawbacks:

- To get menu localization, a plug-in had to set up gettext, even though
  they might want to use something else for their GUI (after all, giving
  facilities for gettext is a good idea, but there is no reason to force
  using this system).
- There was a complex internal system passing the localization domain
  name, as well as the catalog file system path to core, then through
  various classes which we can now get rid of.
- There could be domain name clashes, if 2 plug-ins were to use the same
  i18n domain name. This was handled in now removed functions
  gimp_plug_in_manager_get_locale_domains() by simply keeping a unique
  one (and gimp_plug_in_manager_bind_text_domains() would just bind the
  domain to the kept directory). In o...