Re: [w3c/gamepad] Add buttondown, buttonup, buttonchange, axischange events (#152)

> This looks good. However, I don't see anything about coalescing events as we had discussed. Are we still planning on adding that in?

Considering event coalescing does seem important because I am concerned that the current design will result in lots of events being fired for each "update" of the gamepad. For example, when a control stick is moved diagonally that will generate two axis events with identical timestamps. This seems like it will cause more work to be done on the main thread. It should be possible to handle these events simultaneously. Doing this actually seems like a slightly different problem than what is solved by APIs like PointerEvent.getCoalescedEvents() because in that case the coalesced events would otherwise be discarded while for the Gamepad API they would otherwise be fired next.

I can see a couple ways to avoid this problem:

1. Use a more generic `change` event which provides a list of buttons and axis that have changed.
2. Add a dequeueEvents() method to Gamepad which will return the GamepadAxisEvent and GamepadButtonEvents which will be dispatched next according to the algorithms defined here and perhaps cancel dispatching them.

Maybe the issue only really affects axis (since movement of a joystick with X and Y axis means that there is a high likelihood of paired events) and so only applying proposal (1) to GamepadAxisEvents would be enough and GamepadButtonEvents can be left as specified here. This would also make GamepadAxisEvents similar to PointerEvents in having both X and Y axis (or an arbitrary number of axis) updates in the same event.

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