Code review comment for lp://qastaging/~attente/libindicator/gicon-serialization

I'm a little confused on why we want this. It seems like sending PNGs over the bus is a bad idea because if we do that it'll be impossible to match the theme/resolution on the other side of the bus. I think for this to land we also need a way to communicate that information to the indicator service. This patch should really include that information mechanism as well.