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

[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.

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

Changes: https://git.openjdk.java.net/jdk/pull/710/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=710&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248320
  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