[css3-multicol] Moving the break-* properties to css3-break

Hi,

css3-multicol defines break-before, break-after, break-inside, orphans 
and widows properties.

css3-break defines the same properties with a super-set of the values, 
and great details in prose on how they work.

I believe that css3-multicol should remove this section and reference 
css3-break instead, like we recently did in css3-page.

Cheers,
-- 
Simon Sapin