i have factored out some stuff into convertChoiceIdToChoice that is overridable. overriding convertValue() directly would be cumbersome as you need to mess with string arrays, etc.
basically this is useful for a direct id->choice lookup because right now our ichoicerenderer does a linear O search, and if you have 100+ entries it will get slow as you need to load each choice and then retrieve its id for comparison.
johan what do you think? any objections? it would be nice if this was part of ichoicerenderer, then we can put the linear lookup into choicerenderer implements ichoicerenderer.