The problem was we were using a tap gesture to stop the motion of the
drawer and a drag gesture to settle it. That can cause a broken
lifecycle. Now we use a single drag recognizer to drive the whole
lifecycle.

Fixes #775
Fixes #1276

There's no reason to make clients supply a PointerRounter and a
GestureArena when constructing gesture recognizers. These objects are
statics and the gesture recognizers can just grab them directly.

Also, remove the callback constructor arguments. Almost no code used
them. Instead, people seem to prefer using the `..` operator to set
callbacks on the recognizers. Removing the arguments removes a bunch of
boilerplate.

This allows us to adjust exactly which gestures we're listening for
during layout, which I'll use to kill a SizeObserver.

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

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.

Make all the *GestureRecognizer classes inherit from a class called
GestureRecognizer. Give the old GestureRecognizer a name that is more
precise about its purpose. Remove the members of GestureArenaMember that
aren't used by GestureArenas.

Fixes #1807

Now we use one velocity tracker per pointer.

Path by Kris Giesing.

Fixes #1704

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

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.

Conflicts:
	sky/packages/sky/lib/gestures/drag.dart

Conflicts:
	sky/packages/sky/lib/gestures/drag.dart

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.

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.

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().

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.

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.