Since these are demos, for the sake of showing how the localization
works, let's localize the goat-exercises with a locally installed
catalog.
Note that actually use the gimp30-std-plug-ins catalog, simply I copy it
in the plug-in folder and rename it as org.gimp.extension.goat-exercises
domain.
As a consequence:
- The C plug-in does not need the INIT_I18N anymore, which was
specifically for the centrally installed catalog and cannot be used by
third-party plug-in developers (so it's not a good demo code).
- I now use GLib.dgettext() for Python instead of the gettext Python
module, because the later won't "catch" the catalog declared in
libgimp.
- The other Goat exercises are now localized correctly, unlike before.
- Just setting GETTEXT_PACKAGE is apparently enough for the Vala
plug-in.
- Lua is untested. Hopefully the code will work.
The metadata format being appdata, I simply use standard intltool
merging on the XML templates.
For Automake, I use a custom rule instead of INTLTOOL_XML_RULE because
the m4 macro hardcoded $(top_srcdir)/po as translation source directory
(yet we want to use po-plug-ins/ for these). Meson on the other side
already used a custom command.