Make Handle component propagate onBlur, onMouseDown and onKeyDown events

[jonpacker]

Currently if passing these event handler to a Handle component in a custom handle, they will be swallowed by Handle's internal event handlers. This PR simply propagates the events to the passed event handler if they exist.

[jonpacker]

Not sure what the best way to resolve the linter error is here. It's misleading to put these props on the propTypes because the component itself doesn't do anything with them.

[afc163]

ci broken, please check it.

[yoyo837]

Rebase please.