[TIMOB-20497] Remove "rendering artifact" between launch storyboard and first view

With TIMOB-19694 we have implemented support for Storyboard Launch Files and TIMOB-20453 should fix the "rendering artifact" when you do *not* use the Storyboard Launch Files in Titanium 5.2.0 and later. However, if you *do* use a Storyboard Launch File, you will still see a "rendering artifact", in particular when you need to perform some heavy stuff before you open the first Window. Apple's very [idea](https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/LaunchImages.html) of using a Storyboard Launch File is that you can use actual UI components to mimic an "empty state" version of your app UI which then seamlessly transitioned into your actually UI. In Titanium this is not possible because the root view controller can be seen shortly between the Storyboard Launch File and the app's first view (window). I've tried to set the root view's backgroundColor to clear (transparent) but it will simply show as black. So somehow we need to find a way to either continue to see the Storyboard Launch File until the app's first view has been opened or not let iOS dismiss the Launch File before we have opened the first view. *Sample Code*

var win = Ti.UI.createWindow({
  backgroundColor: 'green'
});

// simulating a delay when you need to do heavy stuff before opening the first window
setTimeout(function() {
  win.open();
}, 3000);