IME IM: A few small fixes

* Increase the buffer size from 10 to 17, to match Simple IM
* Don't convert keyval to unicode before passing it to
  composition checker (composition checker takes keyvals).
* Don't try to feed zeroes to _gtk_im_context_ime_commit_unichar().
  Also remember the number of non-zero chars fed to it,
  and only return TRUE when at least one such char was found.
  This prevents IME IM from returning incorrect values when
  buffer happens to contain zeroes.
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