The semantics of 'button' on events differs between PointerEvent and
MouseEvent, whereas they are the same for 'buttons'. Furthermore, 'buttons'
allows developers to determine when multiple buttons are pressed as the same
time.
https://w3c.github.io/pointerevents/#the-button-property
* Rename `disableContextMenu` to `preventContextMenu`
* Change the behaviour of `preventContextMenu` so that `onContextMenu` is still called when the native context menu is prevented.
* Fixes a bug in the cancellation logic. The cancel events are now correctly listened to on the root.
* Fixes cancellation in Safari by using the dragstart event as a proxy for cancellation (i.e., the event dispatched when move-during-press occurs on an anchor tag)
