[css3-fonts] font matching of grapheme clusters

In addition to a few tweaks of the font load events description, I've 
updated the CSS3 Fonts editor's draft to include an explicit description
of font matching for grapheme clusters:

  http://dev.w3.org/csswg/css3-fonts/#cluster-matching

This includes rules for how sequences containing variation selectors
are matched.

Regards,

John Daggett