Provide the Book Ancestors field from Entity API on D7

Problem/Motivation

The Entity API module provided a property/field called "book_properties" with this code:

      $ancestors = array();
      while (!empty($node->book['plid']) && $node->book['plid'] != -1) {
        $link = book_link_load($node->book['plid']);
        array_unshift($ancestors, $link['nid']);
        $node = node_load($link['nid']);
      }
      return $ancestors;

Proposed resolution

Recreate the "book_properties" code as an appropriate plugin.

Remaining tasks

Add the necessary logic.

User interface changes

A plugin exists that displays the Book Properties code.

API changes

TBD

Data model changes

n/a