Bump libxfce4util required version to 4.19.4 when possible

Just a reminder. It's not really necessary (no need to do a libxfce4util release for that), but libxfce4util@ac83bcc2 complements 09a9f417 and d6b211d9 to ensure that the configuration is completely recovered on xfce4-power-manager-settings startup when xfce4-power-manager is not already running. Currently, sleep, shutdown, etc. capabilities can be wrong, which isn't a big deal (closing and reopening the dialog fixes the problem).