Changes the mouseEvent.type of "mousewheel" to "wheel"

The "mousewheel" is not functioning at all in Dart's Chromium.
According to MDN
https://developer.mozilla.org/en-US/docs/Web/Reference/Events
"wheel" is the standard event type for mouse wheel event.

In addition, the deltaX, deltaY, and deltaZ returned by WheelEvent are
are double values, so I use toInt() to circumvent this issue.
Probably in the future it will be better to directly declare the
deltaX,Y as double variables in the Mouse class.
Merge pull request #60 from TuTu/fix-wheel-event

Changes the mouseEvent.type of "mousewheel" to "wheel"