public const string OPTION_REMAINING
If a long option in the main group has this name, it is not treated as a regular option.
Instead it collects all non-option arguments which would otherwise be left in `argv`. The option must be of type g_option_arg_callback, g_option_arg_string_array or g_option_arg_filename_array.
Using g_option_remaining instead of simply scanning `argv` for leftover arguments has the advantage that GOption takes care of necessary encoding conversions for strings or filenames.