* Added support for passing in velocity and offset into Draggable.onDragCompleted.

* Fixed documentation of DragCompletedCallback.

* Spun off previous onDragCompleted breaking changes into new callback called Draggable.onDragEnd.

* Revert "Fixed documentation of DragCompletedCallback."

This reverts commit 069051f5be0d4ec6a1f4b3f072e535ca87e5d740.

* Revert "Added support for passing in velocity and offset into Draggable.onDragCompleted."

This reverts commit 7ef744aa5645429b7bc92527226203ee8bff68ec.

* DraggableDetails constructor is now declared first as per Flutter code style.

* Draggable.onDragEnd will only call back if its widget is currently mounted to the tree.

* Moved "});" in DraggableDetails constructor to new line, vertically aligned with the constructor name, as per Flutter code style.

* Added space between if statement in drag_target.dart.

* widget.onDragEnd call is now formated as per flutter code style.

* Added more details to DraggableDetails documentation.

* Added brackets to if statement block as per Flutter code style.

* Fixed minor nits in DraggableDetails documentation.

* Made DraggableDetails constructor public. Also added documentation for its constructor.