Refs #6288; Changed the following:

- Readability bar now display only the readability score
- Moved advanced readability score to a tinymce popup that opens when
  clicking on the readability bar
- Plugin is now using the score from every tinymce instance and using
  the advanced popup instead of modifying the contents of a portlet
- Optimized readability action in order for it to be called only once
  every second instead of on every keystroke since the readability score
  is useful when blocks of text are written instead of character counts