编写一个单文档应用程序，此程序运行时在客户区绘制一个上半圆形，当鼠标左击一下后则变为下半圆形，再单击则又变为上半圆形.-Write a single document application, this program is running in the client area to draw a semicircular become the next semi-circular, when the mouse left-click, and then click again changed