Make the buttons for an item visible without hovering if the item is currently empty. This makes it easier to discover items like the rich text item of a textOnly area when they don't have any content to draw the eye yet. We set the apos-empty class on all items that are empty.

TODO: think about the performance hit involved in determining the emptiness of a really big rich text; that algorithm tries to be forgiving of things like stray br's and still treat the item as empty, which means parsing HTML. We could address it by determining emptiness at save time.