[css3-ui] Moving pseudo-classes and pseudo-elements to the Selectors spec

Hi,

Section 4 of css3-ui defines a few selector pseudo-classes and 
pseudo-elements.

The pseudo-classes’ definitions (but at this point not the 
pseudo-elements’) are now also in Selectors 4:
http://dev.w3.org/csswg/selectors4/#ui-states-pseudos

To avoid divergence in the future, only one definition of everything 
should be kept. I’d rather have all selectors defined in the Selectors 
spec, but this is not a strong opinion.

If the pseudo-classes are removed from css3-ui (since they are in 
Selectors), the pseudo-elements should move too. (But see 
http://lists.w3.org/Archives/Public/www-style/2013Jan/0286.html )

-- 
Simon Sapin