RFR: 8223378: CSS solution for navbar problem with <'a> elements is not ideal

[Hannes Wallnöfer]

Please review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8223378
Webrev: http://cr.openjdk.java.net/~hannesw/8223378/webrev.01/
API docs: http://cr.openjdk.java.net/~hannesw/8223378/api.01/

This implements the CSS Flexible Box Layout Module based page structure for the fixed header.
I added a new PageContent builder class to simplify generation of the new layout structure across all the writer classes. In the process I also cleaned up some code a bit (getting rid of unused fields and parameters, removing obsolete javascript etc).

Thanks,
Hannes