Fix make dist errors.

Review URL: https://codereview.appspot.com/8850043
Use EXTRA_DIST instead of DIST_SOURCES.

Review URL: https://codereview.appspot.com/8698047
Implement embed_preedit_text.

BUG=http://code.google.com/p/ibus/issues/detail?id=1606

Review URL: https://codereview.appspot.com/8112044