Implement reliable getLibraryFolder() and getDocumentsFolder() methods in MacPlatform

Created by: benfry

With the removal of FileManager in JDK 9+ we no longer have a means of getting the Documents and Library folders on macOS.

For now, it's possible to use System.getProperty("user.home") and tack Library or Documents onto the end of that, however I'm guessing that it won't be possible in future OS releases, or perhaps even with a signed version of the app, due to security-related changes to how file system access works.

(Appending these folder names works now even on localized systems, since the native language shown is not what's used internally by the OS.)

首先定位 MacPlatform 及其 getLibraryFolder() 和 getDocumentsFolder() 方法。研究用于在不使用 FileManager 的情况下解析这些文件夹的受支持 macOS API，然后在本地化系统和已签名应用程序中验证结果；完成的标准是，在当前 macOS 版本上，这两个方法都能可靠地返回预期的文件夹。