The revision selector didn't really work with touchscreens. Though touch

events will generally emit mouse events, they don't really work

correctly for this widget. To make this work smoothly, we need to handle

touch events.
This change adds event handlers for touch events. These work the same

way as the mouse events, with different elements and related events

being managed depending on the initial event. The mouse and touch event

handlers wrap common methods that do all the usual work.

Unit tests pass.
Tested various drag operations using both the mouse and touchscreens

(real on iOS and simulated in Chrome and Firefox).