RFR: 8189405: More cleanup in HtmlWriter

[Kumar Srinivasan]

Jon,

Looks ok to me, nice simplifications, this affects more on the rendering
side. Bhavesh will likely review it in detail, for the record, I did perform
a before/after test and it looks identical.

Thanks

Kumar

> 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