Fix and rename Config::find to Config::filter

[markuspg]

I always scratched my head looking at Config Config::find(const string &pro), wondering about its purpose. If I don't misinterpret it it creates a new instance of Config - holding all default ConfigItems and then adds the ones of the current instance additionally which fit the passed protocol.

Since I guess it should rather filter I modified the Config constructor to allow creating an instance without any ConfigItems and then adding just selected ones within the filter function.

[markuspg]

I'm not really satisfied with the default argument approach. An alternative would be to leave the constructor unchanged and just call a clear on items before entering into the below for loop to start off from scratch.