Updated Simplified Chinese translation for qttranslations

Hi Qt team. My name is Tyson Tan, a translator from the KDE community. I have filled in a bunch of the strings in the Simplified Chinese (zh_CN) translation of qttranslations.

I apologize for committing my contribution via JIRA which seems to be not the optimal way, but I'm not really a technical personnel and I was unable to figure out how to work with Git with the time I have, so I went this way.

I contributed by downloading the .ts files from Qt's transifex project and uploaded them back once my work was done.

In all, the translated percentage is now 56.63%. It was about about 25% before. I mainly worked both of the 5.12 and 5.15 branches on the following:

• assistant (100%)
• designer (100%)
• linguist (100%)
• qtbase (80%)
• qt_help (100%)
• qtlocation (100%)
• qtmultimedia (100%)
• qtquickcontrols (100%)

I left out the ones I have no idea of. And corrected many mistakes of the graphics related strings which is what I'm specialized in. Since transifex seems to automatically apply new translations to all exact matched strings, the ones I didn't actively worked on should benefit from it as well.

In case the stuff on transifex not being useful, I've attached the files to this report.

If my contribution is accepted by Qt here, do I still need to commit these changes separately to the Qt5-KDE branch?