OSX: Expose 3-finger swipe events in browser-window

[jwheare]

This adds a new swipe event to browser-window for 3-finger trackpad swipes on OS X.
#4181 added support for 2-finger scroll events on OS X, which allow apps to handle some forms of back/forward navigation. However, OS X also exposes a swipeWithEvent method on NSResponder for 3-finger swipes, and it's possible to set these as the "Swipe between pages" gesture. These swipes are only directional, with no velocity, so are easily exposed as one-off events. They will only fire if the option is set in System Preferences to Swipe with two or three fingers or Swipe with three fingers

[zcbenz]

I think we should probably add just one swipe event and pass deltaX and deltaY as parameters.

[zcbenz]

Should use 2 spaces as indention.

[jwheare]

Given the deltas will only ever arrive in 4 combinations, should the API just expose a single direction string instead of the raw deltas?

[jwheare]

I started rewriting this PR to expose the deltas in the API, then got annoyed with having to reimplement the delta checking at the application level, and thinking about everyone else who would have to do the same. I suspect optimising for a more intuitive API vs exposing the underlying mechanism makes more sense in this case.

That said, I agree with collapsing 4 distinct events down to one event with an enum argument; the main reason I didn't initially was to avoid dealing with passing around arguments in the internals.

[jwheare]

I pushed a version with a single direction argument, but I have one with deltas in a separate local branch that I can push instead if you'd prefer.

[zcbenz]

I'm good with a single direction argument, thanks for this!