Reintroduce BUrlResult and add BDataRequest

* BUrlResult is back, with ContentType and Length methods.
* BHttpResult subclasses it and use HTTP header fields to implement
those
* Introduce BDataRequest for "data" URIs. These embed the data inside
the URI, either as plaintext or base64 encoded.
Update HaikuWebkit to version 1.2.1.

* This fixes the crash on trying to download something, and has some new
features as well.