plug-ins: properly localize core Python plug-ins.

- Set the "gimp30-python" Gettext domain and bind it to the proper
  locale directory as installed by GIMP.
- Localize various strings with gettext.
- Remove calls to self.set_translation_domain() in
  do_query_procedures(). This is technically wrong right now but I am
  going to get rid of the menu item localization for plug-ins done by
  the core.

plug-ins/python/file-openraster.py

@@ -454,9 +454,6 @@ def load_ora(procedure, run_mode, file, args, data):
 class FileOpenRaster (Gimp.PlugIn):
     ## GimpPlugIn virtual methods ##
     def do_query_procedures(self):
-        self.set_translation_domain("gimp30-python",
-                                    Gio.file_new_for_path(Gimp.locale_directory()))
-
         return [ 'file-openraster-load-thumb',
                  'file-openraster-load',
                  'file-openraster-save' ]