RFR: 8370446: Support dialogs with StageStyle.EXTENDED (Preview) [v5]

[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 pull request now contains nine commits:

 - Merge branch 'master' into feature/extended-dialog
   
   # Conflicts:
   #	modules/javafx.controls/src/test/java/test/javafx/scene/control/DialogPaneTest.java
 - fix incorrect button metrics
 - 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: https://git.openjdk.org/jfx/pull/1943/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=04
  Stats: 397 lines in 6 files changed: 350 ins; 15 del; 32 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