systemColor() function and support changing accent color in macOS themes

[DevCharly]

This PR adds a systemColor() function to FlatLaf properties files, that can be used to change accent color (or other colors). It is also a preparation for getting accent color from operating system later.

Syntax: systemColor(name[,defaultValue])

Parameters:

• name: system color name (this can be any name)
• defaultValue: default color value used if system color is not available

E.g.

@accentColor = systemColor(accent,#00f)

Where does FlatLaf get the system color from?
Well, it does not get it from the operating system 😞
You have to define a getter function that is invoked when a systemColor() function is found in FlatLaf properties.
E.g.:

FlatLaf.setSystemColorGetter( name -> {
    return name.equals( "accent" ) ? Color.red : null;
} );

This must be done before invoking FlatLightLaf.setup().

As soon as light/dark switching feature is implemented (sponsors wanted 😉 ),
above can be changed to following, which gets accent color from operating system:

FlatLaf.setSystemColorGetter( name -> {
    switch( name ) {
        case "accent":    return SystemTheming.getAccentColor();
        case "highlight": return SystemTheming.getHighlightColor();
        default:          return null;
    }
} );

This PR also adds support for changing accent and highlight colors in macOS themes (PR #533):