Make SelectItem public so that items don't have to be uniquely named

I could be missing something here, so this pull request might not be valid, but at present all the items within a combo box have to be uniquely named. In my use case I would have to generate a random string in order to do this, rather than just select the MenuItem I just added (it's returned from AddItem).

This patch makes SelectItem public (and patches up calls to maintain the existing event propagation), so that it can be called by a user.