RFR: JDK-8275406: Add copy-to-clipboard feature to snippet UI

[Hannes Wallnöfer]

Please review a change to add a copy-to-clipboard feature to snippets. I took special care to make the feature usable on mobile devices. Sample output can be viewed and tested here:

http://cr.openjdk.java.net/~hannesw/8275406/api.00/java.base/java/lang/Throwable.html#printStackTrace()

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Commit messages:
 - Update TestSnippetTag.java
 - JDK-8223357: Fine-tune alignment
 - JDK-8275406: Add copy-to-clipboard feature to snippet UI

Changes: https://git.openjdk.java.net/jdk/pull/6011/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6011&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275406
  Stats: 178 lines in 14 files changed: 166 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6011.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6011/head:pull/6011

PR: https://git.openjdk.java.net/jdk/pull/6011