multifilesink: add handling of seeking query
For a pipeline writing a playable file, we can use filesink as the pipeline's sink but obviously it would be nice if we could use multifilesink instead for its extra abilities. The reason multifilesink can't be used currently is because it doesnt handle the seeking query that an upstream element such as mp4mux(actually qtmux internally) sends to it and therefore there is an internal data stream error.