Bug 1020847 - Add handwriting IME for Simplified Chinese.

The data structure of `_stokePoints` is confusing. It looks like it try to fit many points of many strokes into an 1d array like this

    [start_x, end_y, point_x, point_y, -1, 0, ( ... next stroke .. )]

Couldn't we do a 2d or 3d array to make the data more tangible? Is there a measurable performance concern?

I don't really mind if we create the internal IME API in a way that the first impl recognized, but at least this deserves a comment in the documentation.