Consider adding DigitalOutputState event to track pulse mode falling edges

[bruno-f-cruz]

If an output is set to pulse mode, the onset of the pulse can be timestamped using the write of the OutputSet register. However, there is currently no way to recover the precise event of when the falling edge occurred. It might be interesting to find a way to report this information. I propose co-opting the existing DigitalOutputState and additionally make it an event that behaves identically to DigitalInputState (ie emits a message on any pin change). This event might be omitted on writes to the same register.