RFR: 8369836: Update HeaderBar API

[Kevin Rushforth]

On Tue, 14 Oct 2025 14:20:17 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> 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`.

Reviewers: @andy-goryachev-oracle @kevinrushforth

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PR Comment: https://git.openjdk.org/jfx/pull/1936#issuecomment-3407477471