RFR: 8233338: FX javadoc headings are out of sequence

[Kevin Rushforth]

As indicated in [JDK-8233338](https://bugs.openjdk.java.net/browse/JDK-8233338), this fixes all of our javadoc HTML headings to conform with the rule that all HTML heading tags in class documents must start with `<h2>` and be nested correctly (i.e., no skipping levels). See [JDK-8219801](https://bugs.openjdk.java.net/browse/JDK-8219801) and [JDK-8220379](https://bugs.openjdk.java.net/browse/JDK-8220379).

We have a few places where we are using `<h1>` tags and several more where we are using `<h3>`. As a result, the build of FX docs via `gradle javadoc` fails with JDK 13. We had one `<h4>` heading that wasn't supposed to be nested with the previous `<h3>` heading, so I made both of the `<h2>` to reflect this.

This fix can go in at any time, but must be done before we can switch the boot JDK to 13 in [JDK-8232064](https://bugs.openjdk.java.net/browse/JDK-8232064), else the build will fail.

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Commits:
 - c5d8372a: 8233338: FX javadoc headings are out of sequence

Changes: https://git.openjdk.java.net/jfx/pull/29/files
 Webrev: https://webrevs.openjdk.java.net/jfx/29/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233338
  Stats: 69 lines in 26 files changed: 0 ins; 0 del; 69 mod
  Patch: https://git.openjdk.java.net/jfx/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/29/head:pull/29

PR: https://git.openjdk.java.net/jfx/pull/29