COAttributedStringChunk: Move 'text' property to an ivar. This fixes Typewriter lagging when you type in to a moderately long note (10k words)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10425 7b3f36aa-e6db-49c1-8b4c-66eac1997e64