Remove payload from HandleUpdate?

[dhardy]

It was at one point used to implement RadioBox but now appears to have no use. It is potentially useful, e.g. examples/async-event could encode its hue and pass via this instead of using an Arc<Mutex>. Some options:

1. Remove
2. Keep as is
3. Replace with Option<Box<dyn Any>>?

[dhardy]

This is now Event::Update, which still supports a payload: u64, but is never used.