Make MouseEvents cancelable

I was reporting the `bubbles` and `cancelable` values back from Flash before recreating `MouseEvent`s on the JS side. Alas, it turns out that Flash was marking most/all of the `MouseEvent`s as non-cancelable.

The fix is to just ignore Flash's `MouseEvent` properties for `bubbles` and `cancelable` and set them appropriately [with defaults, in most cases] on the JS side.

Fixes #434.