Bug 1020847 - Add handwriting IME for Simplified Chinese.

Yes, changed it to `clearHandwritingPad(press)` and added press target management in  function `handlePressStart` in HandwritingPadsManager.js. Note, `clearHandwritingPad` is invoked in a timer,  if current keyboard includes multi-handwriting-IME,  if switch very quickly from one handwriting IME to another,  sometimes, the previous handwriting pad may not be cleared, pay attention to this user case. I will write some comments for it.