Adds a RefreshIndicator widget that supports the Material "swipe to refresh" idiom.

Added an over-scroll gallery demo that shows this widget and the existing OverscrollIndicator.

This initial version has some limitations:

• It always appears at the top and bottom of its scrollable. This should be configurable and it should default to just appearing at the top.
• During the drag phase, the refresh indicator should only animate when the drag is moving.
• While the refresh callback is running, the refresh indicator's arrowhead shouldn't appear.
• It should be possible for the refresh callback to update the indicator with its actual progress.
• The refresh indicator should disappear "when the user navigates away from the refreshing content".
• Currently the duration of overscrolls is extended because all drags end in a fling. Both OverscrollIndicator and RefreshIndicator need to cause the Scrollable's ScrollBehavior to set overscroll fling velocities to zero.

See https://www.google.com/design/spec/patterns/swipe-to-refresh.html