* add import guidance to library-level docs
* add import docs to other packages
* fix review comment
* clarify which libraries aren't meant to be directly imported
This lets it cooperate with other gestures like tap.
The way I implemented this was to refactor the entire Draggable gesture
logic to use a new kind of gesture detector called
MultiDragGestureRecognizer. It works a bit like
MultiTapGestureRecognizer but for drags.
Also some tweaks to the velocity estimator.
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.
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.
