Bug 4509 - Bug 4509 - Makes expand/collapse arrows scalable with font size, adds svg versions for higher res, adds logic for gif fallback

It's possible the svg files will be considered to need some design work, but that had best be done by 

* someone with an eye for design, who
* has working hands

I did the best I could to emulate our very pixelated triangles as svgs, but a graphic designer I am not.