Draggable is now itself a gesture arena member. This means it won't
conflict with other gesture recognisers in the same path.

This also allows variants of Draggable that are triggered by other
gestures.

Also, some cleanup of DoubleTapGestureRecognizer, GestureDetector, and
PrimaryPointerGestureRecognizer.

Also, make MultiTapGestureRecognizer support a timeout for longpress.

Also, make Draggable data be typed.

Also, hide warnings about constructor warnings for now. Analyzer doesn't
support them yet. (Have to do this on a per-line basis)

Directions for future research:
 - animating the avatar (enter/exit transitions)
 - interaction with the navigator (canceling a drag on page navigation, etc)
 - double-tap draggable

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.

Document the Gesture Detector constants.

Remove kEdgeSlop since it's obsolete on Android.

Add a test that verifies that when the first tap is canceled, the second
tap can become the first tap of a subsequent two-tap sequence.

Tap and the othes classes in tap.dart are not really related.

Turns out there were no uses of GestureArenaMember other than those that
use DisposableArenaMember.

Fixes #1807

We don't want to expose these from the gesture package. This patch moves
DoubleTapGestureRecognizer into tap.dart so we can make TapTracker and
TapGesture private to that file.

Don't use a timeout to cancel tap tracking. Track only one primary pointer
and ignore non-primary pointers. Update tests to reflect desired behaviors.

Fixes #1779, #1780, #1781.

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.

After this patch, InkWell is driven by gesture recognizers, which lets us
cleanly cancel splashes when the user actually scrolls.

I've also refactored all the clients of InkWell to use InkWell to detect
gestures instead of wrapping InkWell in a GestureDetector.

Fixes #1271

 * Rename GestureTapListener (and friends) To GestureTapCallback to match the
   other gesture callbacks.
 * Replace "ensureFoo" pattern with ??= operator.

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

These gestures use the new gesture detection system.