Add SELECT widget supporting OPTGROUP

Zope's vocabularies don't contain the information necessary to group a Choice or Reference attribute, so that we can group items in a SELECT drop-down menu into OPTGROUP tags, which will make it look much cleaner. This could be handled fairly simply by adding adapters which provide the necessary information that the vocabularies are missing. I am opening a separate bug for this, since it is not always obvious that an OPTGROUP would be helpful until late in development, and it comes at a surprise that it will take more work than anticipated.