Function request: Google DNS/Custom IP in latency test

[Slept-Warrior]

Currently ss-qt5 can only test latency from local to Shadowsocks server,I hope there are 2 ways of latency test:

1. current one;

2. ping google DNS (or custom IP) thru each shadowsocks connections.

the first one indicates connectability from host to ss server, the new one I propose indicates the real latency from local to the server to the Internet, which might be helpful since GFW started to ban shadowsocks connections.

目前ss-qt5的延迟测试功能测试的是本地至ss服务器的ping，结果仅能代表目前该服务器本身对本地的可连接性。 我希望ss-qt5可以在将来增加一项（轮流）从各ss连接ping谷歌DNS（或自定义IP）的功能，由于目前GFW开始逐渐封禁ss，测试本地--ss连接--外网的可用性和性能越发重要，希望可以添加此功能。