RFR: 8301302: Platform preferences API [v2]

[Michael Strauß]

> Platform preferences are the preferred UI settings of the operating system. For example, on Windows this includes the color values identified by the `Windows.UI.ViewManagement.UIColorType` enumeration; on macOS this includes the system color values of the `NSColor` class.
> 
> Exposing these dynamic values to JavaFX applications allows developers to create themes that can integrate seamlessly with the color scheme of the operating system.
> 
> Platform preferences are exposed as an `ObservableMap` of platform-specific key-value pairs, which means that the preferences available on Windows are different from macOS or Linux. JavaFX provides a small, curated list of preferences that are available on most platforms, and are therefore exposed with a platform-independent API:
> 
> 
> public interface Preferences extends ObservableMap<String, Object> {
>     // Platform-independent API
>     ReadOnlyObjectProperty<Appearance> appearanceProperty();
>     ReadOnlyObjectProperty<Color> backgroundColorProperty();
>     ReadOnlyObjectProperty<Color> foregroundColorProperty();
>     ReadOnlyObjectProperty<Color> accentColorProperty();
> 
>     // Convenience methods to retrieve platform-specific values from the map
>     String getString(String key);
>     String getString(String key, String fallbackValue);
>     Boolean getBoolean(String key);
>     boolean getBoolean(String key, boolean fallbackValue);
>     Color getColor(String key);
>     Color getColor(String key, Color fallbackValue);
> }
> 
> 
> The platform appearance is defined by the new `javafx.stage.Appearance` enumeration:
> 
> 
> public enum Appearance {
>     LIGHT,
>     DARK
> }
> 
> 
> An instance of the `Preferences` interface can be retrieved by calling `Platform.getPreferences()`.

Michael Strauß has updated the pull request incrementally with one additional commit since the last revision:

  Use Optional for convenience methods in Preferences

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1014/files
  - new: https://git.openjdk.org/jfx/pull/1014/files/776dabf3..59dfb2b5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1014&range=00-01

  Stats: 106 lines in 3 files changed: 18 ins; 36 del; 52 mod
  Patch: https://git.openjdk.org/jfx/pull/1014.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1014/head:pull/1014

PR: https://git.openjdk.org/jfx/pull/1014