RFR: 8369836: Update HeaderBar API [v7]

[Michael Strauß]

> The `HeaderBar` control currently has three areas: `leading`, `center`, and `trailing`. Additionally, there's `leftSystemInset` and `rightSystemInset`, which are not RTL adjusted. I've come to the understanding that there is no particularly good reason for this, because every time you would want to use this information for layout purposes, it should also be adjusted for RTL.
> 
> With this in mind, there are three changes for the `HeaderBar` control:
> 1. Rename `leading` to `left`, and `trailing` to `right`, which aligns the terminology with `BorderPane`.
> 2. Adjust `leftSystemInset` and `rightSystemInset` for RTL.
> 3. Make `leftSystemInset`, `rightSystemInset`, and `minSystemHeight` attached properties for `Stage`.
> 
> With this change, the `HeaderBar` control is more semantically consistent and easier to use, and the renamed `left` and `right` areas now show its close relationship with `BorderPane`.

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

  add since tag

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1936/files
  - new: https://git.openjdk.org/jfx/pull/1936/files/7d56f0c5..6cf87e00

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=05-06

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1936.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1936/head:pull/1936

PR: https://git.openjdk.org/jfx/pull/1936