Qt: QDropEvent拖拽事件，拖拽打开文件

参考文章：https://blog.csdn.net/rl529014/article/details/53057577?locationNum=8&fps=1

参考链接：

https://www.jianshu.com/p/48f007c2de09

https://www.cnblogs.com/findumars/p/5636482.html

https://www.cnblogs.com/wangjian8888/p/10272513.html

http://blog.sina.com.cn/s/blog_a6fb6cc90101i7lo.html

https://blog.51cto.com/zhweizhi/1883797

https://www.jianshu.com/p/81107b962387

https://blog.csdn.net/u012790503/article/details/84572591

https://blog.csdn.net/u012199908/article/details/50668906

https://blog.csdn.net/xuguangsoft/article/details/8522184