Allow theming functions to override default grouping

Currently theme_imagefield_image_imagecache_lightbox2() uses $args['rel_grouping'] as the last fallback value before going for no grouping. I could not find a way to se its value by overriding the caller theming functions,
as lightbox2_imagefield_image_imagecache() does not allow to alter the values stored in the $args variable.

It would be useful instead to move $args['rel_grouping'] (which seems unused at the moment if I'm not mistaken) as the first choice and allow theming functions calling lightbox2_imagefield_image_imagecache() to set its value, this way we could pass an explicit grouping value overriding the default one.