I'm not sure this specific incarnation of the test ever crashed, since
the original test depended on user interaction and now works fine, but
just in case, here's a regression test for it so I can close that issue.
This also slightly changes the Widget.toString() output to include the
key since that will make debugging easier.
Dismissable now only depends on GestureDetector.
Added a unit test that verifies that issue #1068 has been fixed. It's commented out for now.
Cleaned up VelocityTracker.cc et al a little.
Convert Dismissable to use the ScrollStart, ScrollUpdate, and ScrollEnd gestures. Support for fling gestures is TBD.
Included a basic unit test that checks that one item can be dismissed with a press-drag-release gesture.
Fixed the scroll gesture recognizer: if the last pointer goes up and candidate recognizers still exist, then reject the gesture.
