Detect utf-16 encoded XML files.

* Increase sniffing rule priority to 0.251, otherwise files are detected
as text/plain (for anything lower than 0.25)
* Add sniffing pattern for utf-16 xml.
Web+: advertise support for XML and WebP

* XML files can be viewed if they have XSLT stylesheets.
* WebP will be decoded in the next haikuwebkit release.
utf16 XML: forgot the ?