Add function for detecting characteristic change

[ChrisPallasch]

Please excuse for choosing "issue",
I'm new to Github.

Would it be possible to add some kind of ".valueUpdatedAndChanged" ?

Why?:
My SmartWatch every circa 1-3sec newly reports it's present, even though nothing happend.
This re-transmission seams to be covered by ".valueUpdated", forcing handling of "update".

Thank you very much
Chris