Should show a notification when system updates are downloaded

Software shows a notification for when new updates become available. If automatic updating is disabled, system updates must first be downloaded, after which point an offline update may be issued.

However, there is no way to know when the download of updates has completed. The only way to know that all updates have been downloaded is to check the Updates tab of the Software window and look for a "Restart & Install" button instead of a "Download" button. This shouldn't be necessary, especially since downloads can be updated even while the Software window is closed. It would be nice if Software could show a notification for when updates have been downloaded, so that it's immediately obvious that the computer can be restarted for an offline update without having to manually check anything.