[webappsec] new editor's draft of UISecurity

https://dvcs.w3.org/hg/user-interface-safety/raw-file/43644c06b379/user-interface-safety.html

I've added a new version of UISecurity which addresses concerns about the
input-protection heuristic for user agents using compositing and adds
language about producing unambiguous xpaths.

Thank you,

Brad Hill