BL-663 Predefine Heading1 and Heading2 styles

Several other changes enable this.
- Define Heading1 and Heading2 in Basic Book.css
- Make the style finder skip files not in the book folder
  (gets rid of heading1 and heading2 from editMode.css)
  (also avoids user applying styles not defined in book's own folder)
  - required a change so that for css files that ARE in the book
     folder, we use those versions
- Make the style finder accept style rules containing
  DIV.bloom-editable