Update zh_CN i18n

[clover-yan]

Hi everyone,

I'm a big fan of Drawing and I noticed some areas in the UI translation that could be improved. I went ahead and finished translating them.

I haven't had a chance to test my changes myself, so please let me know if you encounter any issues with the updated .po file.

~~By the way, if you're thinking of adding my name to the project, please use "Clover Yan." That's the name I prefer, although the .po file says "Khyan Yan" (that's my computer account).~~ (fixed)

Thanks for working on such a cool project!

P.S. I'm not sure what's 'pixbuf', so I kept them untranslated in the strings.