COAttributedString: change the schema a bit to make it easier to add more attributes (COAttributedStringAttribute is now a CSS key/value pair like 'font-weight: bold')

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10287 7b3f36aa-e6db-49c1-8b4c-66eac1997e64