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.)

まず MacPlatform とその getLibraryFolder() および getDocumentsFolder() メソッドを見つけます。FileManager を使わずにこれらのフォルダーを解決するために、サポートされている macOS API を調査し、その後、ローカライズされたシステムと署名付きアプリケーションで結果を検証します。完了の条件は、現在の macOS バージョンで両方のメソッドが意図したフォルダーを確実に返すことです。