plug-ins, libgimp: override set_i18n() for all our core plug-ins.

Hence avoiding the stderr messages. These are going to be localized with
centrally installed catalogs "gimp*-std-plugins", "gimp*-script-fu" and
"gimp*-python".

We now handle core plug-in localizations differently and in particular,
with kind of a reverse logic:

- We don't consider "gimp*-std-plugins" to be the default catalog
  anymore. It made sense in the old world where we would consider the
  core plug-ins to be the most important and numerous ones. But we want
  to push a world where people are even more encouraged to develop their
  own plug-ins. These won't use the standard catalog anymore (because
  there are nearly no reasons that the strings are the same, it's only a
  confusing logic). So let's explicitly set the standard catalogs with
  DEFINE_STD_SET_I18N macro (which maps to a different catalog for
  script-fu plug-ins).
- Doing something similar for Python plug-ins which have again their own
  catalog.
- Getting rid of the INIT_I18N macro since now all the locale domain
  binding is done automatically by libgimp when using the set_i18n()
  method infrastructure.

plug-ins/python/python-console/python-console.py

@@ -343,6 +343,9 @@ class PythonConsole (Gimp.PlugIn):
         self.runmode = runmode
 
     ## GimpPlugIn virtual methods ##
+    def do_set_i18n(self, name):
+        return True, 'gimp30-python', None
+
     def do_query_procedures(self):
         return [ PROC_NAME ]