Bug#763882: libgtk-3-bin: Creates cache files in /usr

[Santiago Vila]

Package: libgtk-3-bin
Version: 3.12.2-3+b1

After upgrading a bunch of machines today, I realized that
gtk-update-icon-cache-3.0 creates cache files in /usr.

AFAIK, cache files belong in /var, not in /usr.

I agree that gtk-update-icon-cache may not decide by itself where to
put such files if there is another application that looks for it there,
so please reassign/clone as appropriate.

Thanks.