RFR: 8370446: Support dialogs with StageStyle.EXTENDED [v2]

[Michael Strauß]

> Adds the `Dialog.headerBar` property, which allows developers to specify a custom `HeaderBar` when the dialog uses the `EXTENDED` stage style. The property is ignored for all other stage styles.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  move HeaderBar to DialogPane

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1943/files
  - new: https://git.openjdk.org/jfx/pull/1943/files/52360ffb..c85e8bb6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=00-01

  Stats: 274 lines in 6 files changed: 133 ins; 125 del; 16 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