a couple usability suggestions

Hello, our team at NNELS just wanted to pass along a couple suggestions that may help newbie publishers get more out of the Ace app (we use & recommend Ace a lot, thx!).

- present a warning on the first tab if there is an empty alt tag, i.e. something like 'please review alt text'... (not everyone tabs over to Images tab)
- prompt the user to review the book's headings, as they could be all messed up in a book and yet no one would know (many people assume all is good if some text is listed in the headings and toc)

Sometimes when a book seems to pass everything with 0 violations, publishers don't look much further than that, so having some prompts to take a careful look at alt text and headings may be helpful to guide them