编写一个单文档应用程序，此程序当鼠标单击后在用户区显示一个方形，当用鼠标左击一下后在此方形内出现一内切圆形，再左击鼠标则全部消失。-Write a single document application, the program displays a square in the user area when the mouse click, left-click after a cut inside round, square and then left-click the mouse disa