nsITextToSubURI in Firefox 3.0

 Unescapes only non ASCII characters in the given URI fragment 
 note: this method assumes the URI as UTF-8 and fallbacks to the given charset
 in case the data is not UTF-8

 @param aCharset the charset to convert from
 @param aURIFragment the URI (or URI fragment) to unescape
 @return Unescaped aURIFragment  converted to unicode
 @throws NS_ERROR_UCONV_NOCONV when there is no decoder for aCharset
         or error code of nsIUnicodeDecoder in case of conversion failure

 Unescapes the given URI fragment (for UI purpose only)
 Note: 
 <ul>
  <li> escaping back the result (unescaped string) is not guaranteed to 
       give the original escaped string
  <li> In case of a conversion error, the URI fragment (escaped) is 
       assumed to be in UTF-8 and converted to AString (UTF-16)
  <li> Always succeeeds (callers don't need to do error checking)
 </ul>

 @param aCharset the charset to convert from
 @param aURIFragment the URI (or URI fragment) to unescape
 @return Unescaped aURIFragment  converted to unicode