MFC实现的拖动时图标变成半透明的副本，仿照Win7拖动图标时的样式。代码简单，但愿能起到抛砖引玉的作用。(When the MFC implements drag, the icon becomes a translucent copy, modeled after the Win7 drag icon. The code is simple, I hope to play a role in attracting jade.)