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.)

Inizia individuando MacPlatform e i relativi metodi getLibraryFolder() e getDocumentsFolder(). Studia le API macOS supportate per determinare queste cartelle senza FileManager, quindi verifica il risultato su sistemi localizzati e con applicazioni firmate; il lavoro è completato quando entrambi i metodi restituiscono in modo affidabile le cartelle previste nelle versioni attuali di macOS.