RFR: 8367007: javadoc generation of JavaFX docs fails after fix for JDK-8350920

[Hannes Wallnöfer]

Please review a change to fix a regression when documenting inherited JavaFX property members after [JDK-8350920](https://bugs.openjdk.org/browse/JDK-8350920). The wrong `PropertyHelper` instance was used to initialize synthetic doc comments on property members, leading to potentially missing comments.

Since using the correct `PropertyHelper` instance would have led to property info being computed multiple times (previously it was only needed in `ClassWriter`), I added caching of `PropertyHelper` instances in the enclosing `PropertyUtils` instance. In the process I also removed some unnecessary fields from `PropertyHelper`, made the property member map lazily initialized, and cleaned up code and doc comments in `PropertyUtils` a bit.

The test adds a new subclass to a property-holding class in `TestJavaFX` to make sure inherited property members are documented correctly and no warnings are issued for missing synthetic doc comments.

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Commit messages:
 - 8367007: javadoc generation of JavaFX docs fails after fix for JDK-8350920

Changes: https://git.openjdk.org/jdk/pull/27168/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367007
  Stats: 104 lines in 5 files changed: 53 ins; 13 del; 38 mod
  Patch: https://git.openjdk.org/jdk/pull/27168.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27168/head:pull/27168

PR: https://git.openjdk.org/jdk/pull/27168