Support the legacy api that sends the path to the resolver icon file

Apparently, javascript resolver use the icon api, too, So commit 
https://github.com/tomahawk-player/tomahawk/commit/80bb0de53a06b10787379c53bb0da8521efa7724
broke this api.  Id like to change the resolvers in tomahawk-resolvers and revert this commit one day. Thoughts?