Repo index download no longer shows progress

With the change to the new DownloaderService in 5e59f812, the progress listening in UpdateService no longer works. It is listening for broadcast events from the DownloaderService, but that service is not responsible for downloading the index (nor should it be). Rather, the Downloader is invoked directly by the UpdateService.

Note that there is some progress shown during a repo update, but that is the progress of reading/processing the unzipped index.xml file.

My suggestion is to reinstate the Listener pattern for the Downloader class so that UpdateService can use it. Right now it is there, but documented to only be used by the download service. It seems that global broadcasting of events to whoever is interested is not the right tool for a single method in UpdateService that interacts with the Downloader. However, I wont object if somebody comes up with a way to achieve the progress events as desired with broadcast events.