Add a mime type for XHTML and make Web+ the preferred app.

* WebPositive testsuite relies on the mimetype to parse xhtml files.
* This has a higher priority than HTML, because it is more reliably
detectable
* Uses the same icon as html and mhtml. Who wants to draw new ones?
XHTML sniffing rule: handle utf-16 documents.
ffmpeg: declare support for webm and 3gpp.

* Decoding works on the videos I tested with.
* Some videos on Youtube use these formats.
HttpRequest: handle 302 and 307 redirects.

* Makes jamendo.com player work, as their soundfiles are behind a
temporary redirect for load balancing.
FileRequest: identify unknown files.

* WebKit testsuite relies on the MIME types being correct, so when the
file doesn't have one, try to identify it.
* May be useful for other apps using FileRequest, anyway.