Converts a string from UTF-8 to the encoding GLib uses for filenames.
Note that on Windows GLib uses UTF-8 for filenames; on other platforms, this function indirectly depends on the current locale.
| utf8string | a UTF-8 encoded string. | 
| len | the length of the string, or -1 if the string is nul-terminated. | 
| bytes_read | location to store the number of bytes in  the input string that were successfully converted, or 
                  null.  Even if the conversion was successful, this may be  less than  | 
| bytes_written | the number of bytes stored in the output buffer (not including the terminating nul). | 
| The converted string, or null on an error. |