add new Event to PackageDownloader: "MasterManifestDownloaded" ,this will be used for:
- avoiding case where auto-download ZK can't start because ZK tag is not in the manifest (is empty).
- update busy icon in Rapid Tab when user press reload button.
DownloaderTab: removed (recently added) notification saying "take time to load", seem not working. Show busy icon when Reload button is press (cosmetic tweak)