Add additional selector types to annotations on VitalSource books. These
selectors will be used to:

 - Order annotations in the sidebar by location in the book, when
   sorting by location.
 - Group annotations under headings for the chapter, page or other
   segment in the sidebar
 - Determine which annotations can be anchored in the current chapter,
   so we don't incorrectly mark annotations for chapters other than the
   current one as orphans.
 - Trigger a navigation when the user clicks on an annotation in a
   chapter other than the one currently displayed