RFR: 8370446: Support dialogs with StageStyle.EXTENDED (Preview) [v6]

[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 incrementally with one additional commit since the last revision:

  skip preview feature check for internal use

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1943/files
  - new: https://git.openjdk.org/jfx/pull/1943/files/ccdf88f7..b22712f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=04-05

  Stats: 10 lines in 1 file changed: 5 ins; 0 del; 5 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