Added StartPage_OnDrop() that allows users to drop files on StartPage

Fixes:http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4204
@Benglin Please help me to review it again . 
I think I originally said ```var files = e.Data.GetData(DataFormats.FileDrop) as string[];```. Hard casting it will not result in ```null``` but it will crash the application.