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.
------------- Commit messages: - document/verify preview feature - Ensure that DialogPane is always attached to Scene - Lazily initialize Dialog.headerBar - Support dialogs with StageStyle.EXTENDED Changes: https://git.openjdk.org/jfx/pull/1943/files Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1943&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8370446 Stats: 158 lines in 4 files changed: 135 ins; 13 del; 10 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
