Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

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

Fixes #1807

This patch is part of a sequence of patches towards fewer top-level libraries.
In this patch, the gesture libraries are combined into one gestures.dart
library.

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

The gesture is now called "drag". The library name should match.

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.

These files really belong on other libraries.

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.)

We now have separate gestures for horizontal, vertical, and pan scrolling.

Instead, we pump a onScrollUpdate with the offset if there is an offset.

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.