Tweak defaults for Chinese mode taken from the locale

- detect also the locale zh_SG as simplified Chinese
- if the Chinese variant is detected from the locale, set the
  Chinese mode to a mode which prefers a variant instead of
  showing that variant only. I.e., if zh_CN or zh_SG is in
  the locale, set the Chinese mode to 2 (“Show simplified Chinese first”)
  instead of 0 (“Show *only* simplified Chinese”).
  If one wants to show only one variant, one can easily change that
  later, but as a default it is better not to hide one variant, this
  seems to confuse some users who have not yet discovered that these
  options exist (see for example: https://code.google.com/p/ibus/issues/detail?id=1492
  and https://code.google.com/p/ibus/issues/detail?id=1670 )
- if no Chinese locale is set (and the Chinese mode is neither set in
  the table nor in the user settings either), use mode 4 (“Show all Chinese
  characters”) by default. In this case we don’t really know whether
  the table is intended for simplified or for traditional Chinese,
  it makes no sense to just guess simplified Chinese then.