[macos] Support dark title bars on macOS

Under Java for macOS there is currently no standard way to change the title bar mode from regular to dark or transparent. Both are modes supported by the underlying Cocoa framework (NSAppearance). This is desirable to make sure that Java applications can look just as good under macOS as native applications. NetBeans and its Darcula LAF will also benefit from this.

For further details please see JDK-8181910.

Please pay special attention to Alan Snyder's comments and the fact that it is possible to a) define your own NSString to use as constant when the constants are not available in the API version you are using and b) that you can check the API version you are using.

Thanks.