[Shotwell] Adding simplified Chinese translation

[Lucas Beeler]

Hi Tommy,

   Thank you so much for submitting a Shotwell translation! We hope to
make Shotwell available to as many people around the globe as possible
in their native languages, and the efforts of translators like you
help us advance this goal.

   I received your zh_CN po file yesterday. Our Transifex account is
set up so that newly submitted po files are not committed to our
source repository immediately. Instead, we manually inspect each
submitted po file to verify (and correct, if necessary) its metadata
and encoding before we commit it. This helps us maintain the polish
and consistency we aim for in all our Yorba apps.

   I will commit your zh_CN po file today. That said, we generally
don't commit po files that have fewer than 10% of their strings
translated -- because profiling, updating, and committing po files
consumes time that could be used for coding Shotwell!

   Thanks again!

Wishing you all the best,
Lucas