SlidableController.dispatchEndGesture() reports wrong direction for StillGesture

[ptsekov]

For StillGesture events SlidableController.dispatchEndGesture() uses the 'direction' parameter passed to it by _SlidableGestureDetectorState.handleDragEnd() without taking into account which pane is being dragged. When dragging 'endActionPane' to the left to open it SlidableController.dispatchEndGesture() would generate a StillGesture with 'closing' set to true which is incorrect. As a result '_ActionPaneState.handleEndGestureChanged()' would use 'closeThreshold' instead of 'openThreshold'.