public static void setPersistableFolder(@NonNull final PersistableFolder folder, @Nullable final String folderString, final boolean setByUser) {
        putString(folder.getPrefKeyId(), folderString);
        if (setByUser) {
            handleLegacyValuesOnSet(folder.getPrefKeyId());
        }
    }
sets the user-defined folder-config for a persistable folder. Can be set to null
should be called by PersistableFolder class only
main/src/main/java/cgeo/geocaching/settings/Settings.java