Folder: update the folder version

Since the new index would crash old version of the client, we need to
upgrade folder version so they do not load in the client 2.5.0

src/gui/folder.h

@@ -71,7 +71,7 @@ public:
         FolderDefinition *folder);
 
     /// The highest version in the settings that load() can read
-    static int maxSettingsVersion() { return 1; }
+    static int maxSettingsVersion() { return 2; }
 
     /// Ensure / as separator and trailing /.
     static QString prepareLocalPath(const QString &path);