filesystem restructure work.

Add mount/unmountCommonPath, mount/unmountFullPath, getFullCommonPath.

Reimplement identity and save directory code in terms of new commonpath code.

Add internal createRealDirectory function (not exposed, currently).

src/common/apple.h

@@ -38,10 +38,10 @@ enum UserDirectory
 	USER_DIRECTORY_DOCUMENTS,
 	USER_DIRECTORY_DESKTOP,
 	USER_DIRECTORY_CACHES,
+	USER_DIRECTORY_TEMP,
 };
 
 std::string getUserDirectory(UserDirectory dir);
-
 std::string getExecutablePath();
 
 } // apple

src/common/apple.mm

@@ -52,6 +52,9 @@ std::string getUserDirectory(UserDirectory dir)
 		case USER_DIRECTORY_CACHES:
 			nsdir = NSCachesDirectory;
 			break;
+		case USER_DIRECTORY_TEMP:
+			nsdir = NSItemReplacementDirectory;
+			break;
 		}
 
 		NSArray<NSURL *> *dirs = [[NSFileManager defaultManager] URLsForDirectory:nsdir inDomains:NSUserDomainMask];