[event] Add `wheel` event

This PR modifies the `mousewheel` and adds a `wheel` event (basically `mousewheel` becomes an alias of `wheel`) that matches the W3C spec.

The `wheel` event has a `deltaY` payload that requires some math to get right across browsers. But simulating the event would require to do the math in the reverse order if I wanted to make an assertion over it. Does it make sense to do that or should I not test `deltaY`?

I'm sending the PR to get some early feedback. I still need to:
 * [ ] Update the User Guide
 * [ ] Add manual test
 * [ ] Test in older browsers