This commit fixes the following error:

> attempted to install a procedure with a full menu path as menu label,
> this is not supported any longer.

Instead, if we want new items in menu, we should use
gimp_plugin_menu_register(). I add the calls, yet comment them out,
since I don't think we want these various conversion calls into the
provided filters. The proper way for data conversion should be through
babl/GEGL.