Sugar artwork merge

This set merges most of sugar-artwork to sugar. I left the gtk2 theme in sugar-artwork. This is for a couple of reasons:

* It doesn't make much sense to have a separate module just for the theme. I actually think on the long term we want to merge all the modules what I'm starting to call "sugar OS" inside the sugar module. "sugar OS" is to distinguish from the cross-OS javascript libraries. It should make life a bit easier for both contributors and packages.
* I want to start building sugar without gtk2 and sugar-artwork gets in the way because it can only build with support for both gtk2 and gtk3. I could have modified configure.ac to allow disabling gtk2, but I thought it was a good chance to do the merge instead and leave the gtk2 stuff only in sugar-artwork, so that at some point the module will go away.

I used subtree merging to not lose history

https://help.github.com/articles/working-with-subtree-merge

So basically you can ignore everything before the merge commit e35940f012978cf161e26cf33ff41acd96901da5, it's just exactly what it was in sugar-artwork. After that I move stuff around and add it to the sugar build.