Fix bug 944269 - improve Arabic and Persian text rendering

* Normal letter-spacing for locales with jointed text
* Add Droid Arabic Naskh webfont (fix bug 915860)
* Expand font stack for Arabic, Farsi, and Chinese texts

There may be some locales with jointed/cursive/calligraphic fonts to add to the normal letter-spacing list, but I included the top ones I know of.

There may also be some more fonts to add to the stack for other locales. I've only added a common Chinese sans-serif here and even that isn't being declared exhaustively. Most locales with glyphs that Open Sans doesn't cover will continue to use their system's default sans-serif.