RFR: 8367370: Accent color platform preference not updating in macOS 26 (Tahoe)

[Michael Strauß]

1. Replace undocumented `AppleColorPreferencesChangedNotification` with `NSSystemColorsDidChangeNotification`.
2. Replace undocumented `AppleInterfaceThemeChangedNotification` with a KVO registration for `NSApp.effectiveAppearance`.

This fix can be tested using either PlatformPreferencesChangedTest, or Monkey Tester > Tools > Platform preferences monitor.

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Commit messages:
 - observe NSApp.effectiveAppearance
 - use NSSystemColorsDidChangeNotification

Changes: https://git.openjdk.org/jfx/pull/1895/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1895&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367370
  Stats: 32 lines in 1 file changed: 6 ins; 18 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1895.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1895/head:pull/1895

PR: https://git.openjdk.org/jfx/pull/1895