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 still think we should be explicit here, ie sometime like:
```
case 'pause':
case 'error':
  // delete download
case default:
  console.warn('why the heck are we here?');
```
At the very least, we should add comments to be clear what cases this covers.