RFR: 8362095: HeaderButtonMetrics should not be used across toolkit boundary

[Kevin Rushforth]

On Sun, 13 Jul 2025 21:34:42 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

> The record `com.sun.glass.ui.HeaderButtonMetrics` is used by Glass, but also in the FX layer (in `Stage` and `HeaderBar`). This is architectually unsound, as the FX layer should not directly reference the Glass implementation.
> 
> The solution here is to introduce another record that is used in the FX layer, and translate between the Glass/FX types in quantum toolkit.

LGTM

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Marked as reviewed by kcr (Lead).

PR Review: https://git.openjdk.org/jfx/pull/1846#pullrequestreview-3016062562