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.)

Comienza localizando MacPlatform y sus métodos getLibraryFolder() y getDocumentsFolder(). Investiga las APIs de macOS compatibles para resolver estas carpetas sin FileManager y, después, verifica el resultado en sistemas localizados y con aplicaciones firmadas; se considera terminado cuando ambos métodos devuelven de forma fiable las carpetas previstas en las versiones actuales de macOS.