Improvements, mostly related to CSS and especially CSS print formatting

[ndw]

1. Reworked CSS. The css-links parameter no longer exists (or the v:css-links variable). Added user-css-links and use-docbook-css. The way CSS links are generated has been changed. The underlying CSS, especially the print CSS, has been changed.
2. Added an output-media parameter that has the value screen or print depending on the output media target.
3. Added page-style and paper-size paramters for print rendering. These appear as class attribute values on the html element. The document status is also an class attribute value at this level.
4. Added new class attribute values component, division, formalobject, and informalobject to make grouping chapters, appendixes etc, easier.
5. Added CSS support for a draft watermark.
6. Fixed a bug where sidebar was accidentally rendered in the DocBook namespace.
7. Updated Prism.js and Highlight.js versions