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)
Program.cs: tell first-time ZK & Spring download to wait for MainWindow to start in addition to waiting for Rapid List to download. This prevent ZKL in Linux from having invisible download bar.
ops, typo in previous commit
Merge pull request #154 from xponen/zkl_zk_download_tweak

ZKL: Let ZKL wait for rapid list first before downloading ZK (for first time user)