原理:将icon转化为字符,保存为ttf文件
http://fluttericon.com/
https://segmentfault.com/a/1190000017978633
http://www.muliba.net/flutter/2019/03/29/Flutter%E4%BD%BF%E7%94%A8%E8%87%AA%E5%AE%9A%E4%B9%89%E7%9A%84Icon.html
https://juejin.im/post/5d2994b86fb9a07eac06024a