Fix broken font symlinks

Related to #291 (closed)

I'm unsure on whether my changes to the google fonts were strictly necessary, as I think the issue actually was sdk.bst being built on top of basesdk-integration.bst, not sdk-integration.bst.

Additionally I rework the font-conf file structure so as to not stage things that are unneeded, potentially affecting the install instructions.

I believe this should fix (perhaps) #291 (closed), but without testing I'm unconvinced, and GNOME builder seems to be the best method to reproducibly test, so requires a build of GNOME sdk on top of this patch.

Even if the patch doesn't fix (sadly) #291 (closed) I think not having broken symlinks is still a net positive.