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

[Michael Strauß]

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.

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Commit messages:
 - HeaderButtonMetrics refactor

Changes: https://git.openjdk.org/jfx/pull/1846/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362095
  Stats: 64 lines in 7 files changed: 56 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1846.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1846/head:pull/1846

PR: https://git.openjdk.org/jfx/pull/1846