RFR: Merge jfx19

[Kevin Rushforth]

Merge `jfx19` into master.

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Commit messages:
 - Merge jfx19
 - 8290393: Code sample in javadoc of ObservableValue flatMap is incorrect
 - 8290331: Binding value left null when immediately revalidated in invalidation listener

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/jfx/pull/832/files
  Stats: 31 lines in 3 files changed: 26 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/832.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/832/head:pull/832

PR: https://git.openjdk.org/jfx/pull/832