Establish a "fonts" directory in a place where it can be shared between core and contrib themes.

This is a follow-up to #1452 ...

In that issue, we ended up moving OpenSans out of the Seven core theme and into /core/misc/opensans/opensans.css. The original issue though was about shipping core with a /core/fonts and a /fonts directory:

I'm not sure if this is makes sense or if it's worth pursuing at all, but here it is...

I noticed a considerable difference in the filesize of the .zip from the master branch since the last time I had to download it, so I decided to analyze the size of our various directories. I noticed that when extracted, the /core/themes directory takes up almost half the size of the entire distribution. Digging further down revealed that Seven takes up 99% of that space and that is because of the fonts directory. Then I realized that fonts is a resource that various themes might need to access and it might either be a waste to include the same font in more than one themes or simply silly to point to a specific font of a theme from another theme (what happens if the theme that actually ships with the font is removed from the installation?).

So the question here is should we introduce a top-level /core/fonts directory (as well as a /fonts directory for contrib themes)?

...related: #364