bug 1077595 - Use DataStoreChangeEvents to update Download API Manager c...

...ache up to date. Move tracking of active downloads into StatusBar instead of DownloadNotifications. r=crdlc,mhenretty
I was considering wrapping the event further as well. However, I would currently end up using exactly the same data and event names so I didn't really see any point in adding an extra layer of complexity at this time. The intention though, is to keep the events that the download list receive the same, even if the datastore operation names change later on. I've added comments in the code to reflect this.