Follow DTM changes (about change notification)

 * Updated the dom-text-mapper library to 3fb30bee27
 * Followed the API changes: DOM changes are now announced in events,
   and we don't call the dom-text-mapper library directly.
   * When highlights are created and removed
   * When magic range slices and dices the text elements
 * PDF plugin now registers a handler to the same event type, to catch
   cross-page events