removed obsolete configure option

Closes https://github.com/mate-desktop/atril/pull/112
ePub backend: unescape uri strings from toc

Toc may include links with strings escaped for URI format. We needed to un-escape them.
ePub: Can open documents which do not have toc

I always thought all ePub documents had either a traditional ncx or an epub3 nav. Now as it turns out,
sometimes neither of these are present. Fixed.
epub backend: more anti-NULL protection for epub_document_set_index_pages