Fixes #1320

Fixes #1084

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

This fixes an issue in the stocks app in horizontal mode where you could both
scroll and drag the drawer at the same time.

That way the fling engages in the same direction as the scroll. For example, if
you have a horizontal scroll nested inside a vertical scroll, the fling will
take place in the same direction as the scroll.

Alternating scroll gestures would sometimes be ignored because _ScrollGestureRecognizer didn't always reset its _state when the pointer[s] went up.

A Dismissable dismiss triggered by a drag and then a fling could cause the next attempt to drag-dimiss to fail.

Fixed the definition of lerpColor().

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)