Annotator: PDF.js-based strategy is a plugin now

 - Update Annotator to 0c2668ea (pdf-support-2 branch), which has support
   for dynamically registering document access strategies.
 - Add new PDF.jd plugin
 - Remove workaround for manually tinkering with the root node of the mapper
   (I don't remember why was it needed in the first place.)
 - Cosmetic changes