R1:6b5204dca954

Add support for device swipe events

Summary:
Ref T2700. Allow JS to listen for swipes on devices.

There are a bunch of tricky cases here and I probably didn't get them all totally right, but this interaction broadly looks like this:

- We implement gesture recognition for the mouse in device modes (narrow browser), and for touch events from an actual device.
- The sigil `touchable` indicates that a node wants to react to touch events.
- When the user touches a `touchable` node, we start listening…