i18n: add zh_CN support

[samhjn]

Few years earlier when I was an undergraduate student I tried to translate openfluid without Qt i18n framework and I made a promise that I may do it with the Qt framework later. Then I leaved that team and graduated, and rarely kept up with the progress of the OpenFluid project. At times, I still recall the commitment I had made to complete the Chinese translation earlier, but I have been consistently busy with other tasks. Now I found some spare time and tried to work on it. Hope it met your standard on i18n and wasn't too late.

[Arthoni]

Thank you for the time and effort dedicated to this task, we appreciate your help! We will check your proposal and may integrate it at some point.