Support scrolling by dragging

[nightkr]

This is pretty much required for touch-friendly UIs. I've hacked up a wrapper widget that implements this by listening for global events, but it's probably not a great direction in the long term:

• It has read-only access to the events, so the child widgets still get them as well
• Same in the reverse, there is no practical way for child widgets to inhibit the scrolling if they handle the drag
• It requires opting in for each widget to get parity with mouse wheel scrolling