创建一个基于单文档的工程，并为添加一个模态窗口，其过程分为：创建模态窗口，创建窗口类，向程序中添加代码，添加消息响应函数。-Create a project based on a single document, and add a modal window, the process is divided into: Creating modal window, create window class to add code, add a message response function.