Better display of real missing pages in the text

[Martin Mueller]

the missing page display may not be clear enough to casual users. I noticed that the encoding is not consistent either. I'll change it so that every lacuna of a page or more shows up in the XML as <gap unit="pages" extent="2"/>. Then we can figure out a good way of showing this to the user.

This is definitely not a bridge that needs crossing right now.

[Craig Berry]

We now display instructions informing the user the page is blank and to use the navigation buttons or page slider to move to another page.