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.