enhance unicode width detection, adds test 4 utf8

[laelnasan]

Changes to be committed: modified: src/ltui/label.lua new file: tests/utf8dialog.lua

• Before adding new features and new modules, please go to issues to submit the relevant feature description first.
• Write good commit messages and use the same coding conventions as the rest of the project.
• Please commit code to dev branch and we will merge into master branch in feature
• Ensure your edited codes with four spaces instead of TAB.

---

• 增加新特性和新模块之前，请先到issues提交相关特性说明，经过讨论评估确认后，再进行相应的代码提交，避免做无用工作。
• 编写友好可读的提交信息，并使用与工程代码相同的代码规范，代码请用4个空格字符代替tab缩进。
• 请提交代码到dev分支，如果通过，我们会在特定时间合并到master分支上。
• 为了规范化提交日志的格式，commit消息，不要用中文，请用英文描述。

[waruqi]

I have tested it, It works on macOS. I have merge it although utf8 is not yet supported on windows/pdcurses.

[laelnasan]

I've submitted a pr #28 with the pdcurses flag for wide characters. It is a bit old but should at least print some chars