[selectors-api] querySelector with namespace

Hi,

During a discussion about xml:id I was about to make the throw away comment that
you could use querySelector to easily work around lack of support for xml:id,
but on checking it turns out that's not the case. querySelector, it seems,
cannot be used to select on a specific namespace, since you can only use
namespace prefixes in selectors, and querySelector does not resolve prefixes.

Maybe the working group could consider adding some sort of "non-prefix"
namespace support to selectors for the sake of querySelector/querySelectorsAll,
something like:

  [url("http://www.w3.org/XML/1998/namespace")|id="foo"]

or:

  [namespace("http://www.w3.org/XML/1998/namespace")|id="foo"]

or:

  [ns("http://www.w3.org/XML/1998/namespace")|id="foo"]

I imagine this would be much more widely useful than just for xml:id of course.

Jonathan