(rationale: don't let people install @2 in a build system that will not alert them that we have moved to ES6, only to cause trouble with a later release.)

notes:
- needs https://github.com/d3/d3-selection/tree/two-multitouch-pointers d3/d3-selection@e84c95f for dbltap to work
- needs d3-drag@2 also

… ...)

(was a leftover from when I copied event code from d3-drag)

so that zoom.extent(function(event, d) {}) receives *null, datum*

…, …); and repair the calls that were mixed-up.

*event-based functions:*
- Gesture (hence event = this.args[0])
- filter
- wheelDelta

*selection-based functions:*
- extent
- transform
- schedule
- k,p (in scaleBy & scaleTo)
- x,y (in translateBy)

In addition, an event that triggers a transition (such as a zoom.dblclick) can be persisted throughout the transition as the zoomEvent.sourceEvent (fixes #202).

fixes #204
An unintended consequence is that wheel-zooming with the ctrlKey pressed goes 10x faster, but it's rather agreeable.

fixes #192

fixes #180
fixes #206

refs:
- d3/d3-brush#71 (comment)
- d3/d3-brush@4d5cfc0

fixes #206
fixes #217

x and y are scaled by t_k when calling t.translate(x,y)