RFR: 8362091: Window title bar should reflect scene color scheme

[Michael Strauß]

Currently, the color scheme of a system-decorated stage is as follows:
* On Windows, the title bar is always light (even if the OS color scheme is dark).
* On macOS and Linux, the title bar is light or dark depending on the OS color scheme.

The expected behavior is that the title bar matches the color scheme of the `Scene`.
If an application doesn't specify a color scheme, the title bar color should match the OS color scheme.

This PR fixes the behavior for Windows and macOS, but not for Linux (there's no good way to do that).
Depending on how you look at it, this is either a bug fix or an enhancement.

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Commit messages:
 - Window decorations adapt to color scheme

Changes: https://git.openjdk.org/jfx/pull/1845/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1845&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8362091
  Stats: 183 lines in 17 files changed: 171 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jfx/pull/1845.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1845/head:pull/1845

PR: https://git.openjdk.org/jfx/pull/1845