RFR: JDK-8248320: Provide a unique accessible name for <nav role="navigation"> [v2]

[Jonathan Gibbons]

> This change is primarily about removing the lower (bottom) instance of the navigation bar, but as a side effect, it is also a cleanup for the way that the `<header>` and `<footer>` elements are generated for each page.
> 
> The header and footer elements are now generated by two new methods on `HtmlDocletWriter`: `getHeader` and `getFooter`. 
> 
> The footer is the same for all pages, and now just consists of the "bottom" text (if any) provided with the `-bottom` option. (In JDK API docs, this is all the legal text at the bottom of each page.)  The footer is omitted if there is no bottom text. 
> 
> The header is structurally the same for each page, consisting of the "top" text (if any) provided with the `-top` option, and the main navigation bar. The contents of the navigation bar does depend on the kind of page, and so it is generated by a new method `getNavBar`, which is overridden as needed for some pages, to properly customize the properties.
> 
> Since there is no longer a lower navbar, the `-footer` option, which could be used to inject user-provided text into the bar bar, is now redundant. For now, it is accepted but generates a warning if used.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge master
 - introduce getHeader
 - JDK-8248320: Provide a unique accessible name for <nav role="navigation">

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Changes: https://git.openjdk.java.net/jdk/pull/710/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=710&range=01
  Stats: 622 lines in 28 files changed: 111 ins; 354 del; 157 mod
  Patch: https://git.openjdk.java.net/jdk/pull/710.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/710/head:pull/710

PR: https://git.openjdk.java.net/jdk/pull/710