receive icon directly from resolvers

Dont let external resolvers send a (probably faulty) path to an image
file, but receive the image itself directly. This is consistent with
the way the config _msgtype works, so comression is supported, too.

I know this breaks the way current resolvers send their icons, but afaik the only resolvers that used this api were DAAP and Google Music resolvers, which are both maintained by me...