RFR: JDK-8259283: use new HtmlId and HtmlIds classes [v3]

[Jonathan Gibbons]

> Please review this change to centralize the management of HTML ids used by the standard doclet in a single new factory class, `HtmlIds`, which utilizes a new type-safe wrapper class around `String` called `HtmlId`.
> 
> The new classes are used both when declaring ids (e.g. `HtmlTree.setId`) and when referencing them (e.g. `Links.createLink`).  The enum `SectionName`, which declared a set of standard ids, goes away, as do methods in `Links` and other places to create "anchors".
> 
> This is a simple refactoring: there is no intentional change of functionality, and no tests are affected or need to be updated. However, the changes do highlight the inconsistency of the use of ids: it would be good to rationalize these in separate, targeted follow-up work.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  tidy up merge

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1951/files
  - new: https://git.openjdk.java.net/jdk/pull/1951/files/5d32ab5e..2ddcf132

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1951&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1951&range=01-02

  Stats: 9 lines in 2 files changed: 2 ins; 3 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1951.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1951/head:pull/1951

PR: https://git.openjdk.java.net/jdk/pull/1951