Add Chinese Simplified translation.

The [help page](https://crowdin.net/translate/inkpad/50/zhcn-zhcn) is translating. It will be committed after several days.

Translators:
Pillow Tse <https://github.com/xiezhhw>
Zhang Yungui <https://github.com/rhcad>

Proofreaders:
Hu Haonan <https://github.com/huhaonan>