Add an option to play a sound when caps/num/scroll lock is pressed

My keyboard, as well as many others, comes without LEDs on the caps/num/scroll lock keys. Therefore, I have no way of knowing whether I pressed Caps lock by accident, until I start typing. Add a toggle to xfce4-accessibility-settings's Keyboard page, which plays a sound when a lock key is pressed, perhaps a different one for when it is turned on and off.