RFR: 8369836: Update HeaderBar API [v2]

[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`.

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

  Make leftSystemInset/rightSystemInset/minSystemHeight attached properties

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1936/files
  - new: https://git.openjdk.org/jfx/pull/1936/files/137a61e7..da4d219a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=00-01

  Stats: 369 lines in 3 files changed: 231 ins; 116 del; 22 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