Move UrlResult to HttpResult

 * Remove the fRawData field, as handling it is too complicated (it's
not easy to have proper copy semantics on a BDataIO) and it's not used
anyway, as the listener DataReceived call is enough to get the data and
handle it.
 * All the remaining fields are HTTP-only, so rename the class to
HttpResult and attach it to HttpRequest instead of UrlRequest.
Add file: protocol handler.