RFR: JDK-8277420: Provide a way to copy the hyperlink to a doc element to the clipboard

[Hannes Wallnöfer]

This is a CSS/JavaScript only change to implement copy-to-clipboard functionality for all headers (`h1` - `h6`) that have an `id` attribute associated with them. The following element-attribute patterns are supported (using `<h2>` as an example):

 - `<section id="..."><h2>` (generated by javadoc)
 - `<h2 id="...">` (commonly used)
 - `<h2><a id="...">` (legacy)

The change includes a consolidation of the CSS styles used to render copy-to-clipboard buttons, of which we have now three kinds: for snippets, for the link on the search page, and for headers. There is now a base CSS class called "copy" that defines the styles shared by all copy-to-clipboard buttons, and additional CSS properties for the concrete "subclasses". 

API docs generated with this change can be viewed here (top level files and java.base module):
http://cr.openjdk.java.net/~hannesw/8277420/api.02/

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Commit messages:
 - Clean up copy-to-clipboard button styles
 - JDK-8277420: Provide a way to copy the hyperlink to a doc element to the clipboard

Changes: https://git.openjdk.java.net/jdk/pull/8817/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8817&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277420
  Stats: 253 lines in 8 files changed: 150 ins; 96 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8817.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8817/head:pull/8817

PR: https://git.openjdk.java.net/jdk/pull/8817