RFR: 8357067: Platform preference change can emit multiple notifications

[Michael Strauß]

Some platform preference changes can trigger the emission of multiple notifications. For example, when switching from a high-contrast theme on Windows to the regular theme, the following notifications are emitted (log can be viewed in `PlatformPreferencesChangedTest`):


changed:
	Windows.UIColor.Accent=0x0078d4ff
	Windows.SysColor.COLOR_HIGHLIGHTTEXT=0xffffffff
	Windows.SysColor.COLOR_WINDOW=0xffffffff
	Windows.UIColor.AccentLight1=0x0091f8ff
	Windows.SysColor.COLOR_3DFACE=0xf0f0f0ff
	Windows.SysColor.COLOR_HOTLIGHT=0x0066ccff
	Windows.SysColor.COLOR_WINDOWTEXT=0x000000ff
	Windows.SysColor.COLOR_BTNTEXT=0x000000ff
	Windows.UIColor.Foreground=0x000000ff
	Windows.UIColor.AccentDark1=0x0067c0ff
	Windows.UIColor.Background=0xffffffff
	Windows.UIColor.AccentLight3=0x99ebffff
	Windows.UIColor.AccentLight2=0x4cc2ffff
	Windows.SysColor.COLOR_GRAYTEXT=0x6d6d6dff
	Windows.SysColor.COLOR_HIGHLIGHT=0x0078d7ff
	Windows.UIColor.AccentDark2=0x003e92ff
	Windows.UIColor.AccentDark3=0x001a68ff

changed:
	-Windows.SPI.HighContrastColorScheme
	Windows.SPI.HighContrast=false

changed:
	Windows.UIColor.Foreground=0xffffffff
	Windows.UIColor.Background=0x000000ff


Notably, the values for Windows.UIColor.Foreground/Background are inconsistent between the notifications (although they are eventually correct). In general, only a single notification should be emitted that includes all of the changed preferences.

This artifact is only visible on Windows. The reason is that changing some system settings (like high-contrast theme) causes a number of different window messages to be sent to the application. We should wait for all window messages to come in, and then aggregate all of the changed preferences into a single notification.

In order to minimize the delay between changing a system setting and sending out the notification in JavaFX, the implementation only waits when instructed to do so by the native toolkit. This allows us to instantly send out notifications for most changes, but selectively wait a bit for changes where the native toolkit knows that more changes might be coming in.

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Commit messages:
 - add delayed change aggregation

Changes: https://git.openjdk.org/jfx/pull/1810/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1810&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357067
  Stats: 319 lines in 13 files changed: 282 ins; 1 del; 36 mod
  Patch: https://git.openjdk.org/jfx/pull/1810.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1810/head:pull/1810

PR: https://git.openjdk.org/jfx/pull/1810