RFR: JDK-8280402: Add new convenience forms to HtmlTree

[Jonathan Gibbons]

Please review a simple `noreg-clean` update to `HtmlTree` and a couple of friends.  The updates are to continue the trend towards the use of Facebook tory methods for `HtmlTree` objects, and to leverage generic methods and lambdas to generate composite content. The cleanup is mostly extracted from some parallel work for a separate PR.  Some of the more general cleanup was suggested by an IDE.

All javadoc tests pass locally; I have a job in  progress to validate the fix on all platforms, although there is no reason to believe there might be any issues.

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Commit messages:
 - fix whitespace
 - JDK-8280402: Add new convenience forms to HtmlTree

Changes: https://git.openjdk.java.net/jdk/pull/7166/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7166&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280402
  Stats: 174 lines in 3 files changed: 100 ins; 22 del; 52 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7166.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7166/head:pull/7166

PR: https://git.openjdk.java.net/jdk/pull/7166