You will also write navigation to help the user mover forward and backward through the image list.
还要编写导航，帮助用户在图像列表中来回移动。
Pointer to the image list to which the dragging bitmap was added, if successful; otherwise NULL.
如果成功，则返回指向图像列表的指针，拖动位图将被加入到其中去；否则返回NULL。
Pointer to the image list to which the dragging bitmap was added, if successful; otherwise NULL.
如果成功，则返回指向图像列表的指针，拖动位图将被加入到其中去；否则返回NULL。
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