Sorting added to spec

I've added sorting to the relevant parts of the spec (namely https://www.w3.org/community/xformsusers/wiki/XForms_2.0#Property_Attributes, and the references).


I still have to add a bit to <insert/>


Some observations, and questions.


* I updated the reference to XPath functions and operators 3.0, which said it was a candidate recommendation, but has since become a Recommendation.
  I wonder if we should update this to 3.1, especially since we use the collation algorithm from there.


* XPath FOP 3.1 says "If the default collation is not specified by the user or the system, the default collation is the ·Unicode codepoint collation· <https://www.w3.org/TR/xpath-functions-31/#dt-codepoint-collation> ."
  I think we should say that too.


Steven