add PointerID and PointerType to PointerEvent, as well as DataTransfer.SetData

[dmitshur]

I plan to merge this in a few days. This PR exists so that it's possible to leave review comments before then, but a review is not required. Thanks.

References:

• https://developer.mozilla.org/docs/Web/API/PointerEvent/pointerId
• https://developer.mozilla.org/docs/Web/API/PointerEvent/pointerType
• https://developer.mozilla.org/docs/Web/API/DataTransfer/setData

[dominikh]

Please don't use {...} in commit messages like that, they make it more difficult to grep for the names of symbols in the commit log.