RFR: 8313424: JavaFX controls in the title bar [v3]

[Michael Strauß]

> This PR is a new take on a highly requested feature: JavaFX controls in the header bar (see also #594 for an earlier iteration).
> 
> This is a feature with many possible ways to skin the cat, and it has taken quite a bit of effort to come up with a good user model. In contrast to the previous iteration, the focus has shifted from providing an entirely undecorated window to providing a window with a user-configurable header bar.
> 
> The customizable header bar is a new layout container: `javafx.scene.layout.HeaderBar`. It has three areas that accept child nodes: leading, center, and trailing.  `HeaderBar` also automatically adjusts for the placement of the default window buttons (minimize, maximize, close) on the left or right side of the window.
> 
> The customizable header bar is combined with a new `EXTENDED` stage style, which extends the client area into the header bar area. The new extended stage style is supported on Windows, macOS, and Linux. For platforms that don't support this stage style, it automatically downgrades to `DECORATED`.
> 
> This is how it looks like on each of the three operating systems:
> 
> ![extendedwindow](https://github.com/user-attachments/assets/9d798af6-09f4-4337-8210-6eae91079d3a)
> 
> The window buttons (minimize, maximize, close) are provided by JavaFX, not by the application developer. This makes it easier to get basic window functionality without recreating the entirety of the window controls for all platforms.
> 
> ## Usage
> This is a minimal example that uses a custom header bar with a `TextField` in the center area. `HeaderBar` is usually placed in the top area of a `BorderPane` root container:
> 
> public class MyApp extends Application {
>     @Override
>     public void start(Stage stage) {
>         var headerBar = new HeaderBar();
>         headerBar.setCenter(new TextField());
> 
>         var root = new BorderPane();
>         root.setTop(headerBar);
> 
>         stage.setScene(new Scene(root));
>         stage.initStyle(StageStyle.EXTENDED);
>         stage.show();
>     }
> }
> 
> To learn more about the details of the API, refer to the documentation of `StageStyle.EXTENDED` and `HeaderBar`.
> 
> ## Platform integration
> The implementation varies per platform, and ranges from pretty easy to quite involved:
> 1. **macOS**: The window buttons are provided by macOS, we just leave an empty area where the window buttons will appear. The client area is extended to cover the entire window by setting the `NSWindowStyleMaskFullSizeContentView` flag. A click-and-drag operation is ini...

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

  revert unintended change

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1605/files
  - new: https://git.openjdk.org/jfx/pull/1605/files/0ddd63d4..79696225

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1605&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1605&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1605.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1605/head:pull/1605

PR: https://git.openjdk.org/jfx/pull/1605