RFR: 8370446: Support dialogs with StageStyle.EXTENDED [v3]

[Michael Strauß]

> Adds the `DialogPane.headerBar` property, which allows developers to specify a custom `HeaderBar` when the dialog uses the `EXTENDED` stage style.

Michael Strauß has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - documentation
 - Merge branch 'master' into feature/extended-dialog
 - move HeaderBar to DialogPane
 - document/verify preview feature
 - Ensure that DialogPane is always attached to Scene
 - Lazily initialize Dialog.headerBar
 - Support dialogs with StageStyle.EXTENDED

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1943/files
  - new: https://git.openjdk.org/jfx/pull/1943/files/c85e8bb6..1cc864de

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=01-02

  Stats: 1592 lines in 50 files changed: 1245 ins; 133 del; 214 mod
  Patch: https://git.openjdk.org/jfx/pull/1943.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1943/head:pull/1943

PR: https://git.openjdk.org/jfx/pull/1943