Implement canceling for event-less callbacks (e.g. onPress)

[mkrause]

There are are certain callbacks like onPress which do not take an Event instance. There is no way to cancel the rest of the event handler chain. It may be good to enable this. See for example SegmentedControl, where there is no way to stop the inner onPress handler from running.

Possibly also useful for event handlers that do take an Event? Because event.stopPropagation() right now is not respected by our own components, only by the browser.

Question: since there's no Event argument, how do we communicate this? Options:

1. Add a (first) argument to all these callbacks that takes a basic Event-like argument.
2. Use this: this.cancel().