Issue #1006: JavaFX demo — embed chart in a VBox with other controls; fix Apple Silicon

[timmolter]

Addresses the question in #1006: how to embed an XChart inside a JavaFX layout container (e.g. a VBox with a ButtonBar) so everything resizes with the window.

Changes

JavaFXDemo.java — now demonstrates the realistic embedding case from the issue:

• The chart's SwingNode is wrapped in a StackPane marked VBox.setVgrow(..., Priority.ALWAYS) — the key line, since a VBox only gives children their preferred height by default and XChartPanel reports a fixed preferred size.
• A ButtonBar with a Close button sits below the chart; the chart absorbs all resize space while the buttons keep their natural height.
• Swing content is now built on the EDT via SwingUtilities.invokeLater, as the SwingNode javadoc requires.

xchart-demo/pom.xml — JavaFX bumped 11.0.2 → 17.0.20 (demo module only):

• 11.0.2 ships x86_64-only macOS natives, so the demo crashed with UnsatisfiedLinkError on Apple Silicon; mac-aarch64 artifacts start at 17.0.2.
• 17.x is the newest LTS line that still runs on Java 11 (JavaFX 21 raised the minimum JDK to 17), so this keeps the project's Java 11 baseline intact.

Testing

Verified manually on an Apple Silicon Mac with mvn javafx:run -pl xchart-demo: window opens, chart fills the space above the button bar, and both resize correctly with the window.

Closes #1006

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