This package contains WidgetTester, which is very useful when writing tests for
widgets.

- force the time dilation to 1.0 for the Widget tests, so that a local
  change doesn't break all the tests during development.
- add missing license block to all the files.
- set ui.window.onBeginFrame to null when you use WidgetTester, so that
  the engine doesn't trigger any confusing frames after our fake frames.

This seems useful for numbering the entries in the list, for example.

Fixes #328

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.

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.

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.