Please review a medium-simple fix to continue the HtmlWriter cleanup
started in JDK-8160697.
This is about removing "constants" from the per-page HtmlWriter, such
that they are either moved
to the shared Contents class, or just created as needed.
This is done in part by introducing a new TableHeader class to help
create headers for tables.
This is cleanup only: no new tests or change in generated docs.
JBS: https://bugs.openjdk.java.net/browse/JDK-8189405
Webrev: http://cr.openjdk.java.net/~jjg/8189405/webrev.00/
-- Jon
