RFR: 8369836: Update HeaderBar API [v6]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - review changes, headerbar properties
 - Merge branch 'master' into headerbar-update
 - stage doc
 - Merge branch 'master' into headerbar-update
 - documentation
 - Merge branch 'master' into headerbar-update
 - Make leftSystemInset/rightSystemInset/minSystemHeight attached properties
 - Update HeaderBar API

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1936/files
  - new: https://git.openjdk.org/jfx/pull/1936/files/02b1150c..7d56f0c5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1936&range=04-05

  Stats: 2544 lines in 75 files changed: 1975 ins; 272 del; 297 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