Add support for saving Whiskermenu configuration file

The settings for xfce4-whiskermenu-plugin will now be saved during the panel backup process and be properly restored. If there is no configuration file on the user's machine (.config/xfce4/panel/whiskermenu-1.rc), it will not be saved in the backup. If there is no configuration file in the backup (whiskermenu-1.rc), it will not throw an error and will just be ignored.

I haven't tested every possible configuration with these changes but everything I've tried works. Hope this can be incorporated soon!