Bug 1020847 - Add handwriting IME for Simplified Chinese

I don't think we should send `layoutName` to the IMEngine. I think we should introduce a `handwritingLanguage` property on the layout and send it as the first argument. That makes the line 437 into something like

    var lang = this.app.layoutManager.currentPage.autoCorrectLanguage || 
        this.app.layoutManager.currentPage.handwritingLanguage;
    imEngine.activate(lang, ...)

If you could come up a name that generalize `autoCorrectLanguage` and `handwritingLanguage` that would be great too!