RFR: 8369836: Update HeaderBar API

[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 two 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.

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`.

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Commit messages:
 - Update HeaderBar API

Changes: https://git.openjdk.org/jfx/pull/1936/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369836
  Stats: 212 lines in 3 files changed: 14 ins; 56 del; 142 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