Support launch storyboards as splash screens on iOS

[ryanmasondavies]

Apple's docs recommend the use of storyboards as launch screens:

Because device screen sizes vary, launch screen sizes vary too. To accommodate this, you can provide a launch screen as an Xcode storyboard or as a set of static images for the devices your app supports. Using an Xcode storyboard is the recommended approach, as storyboards are flexible and adaptable. You can use a single storyboard to manage all of your launch screens. To learn about implementing adaptable interfaces, see Auto Layout Guide.

This PR changes -[RCCManager showSplashScreen] so that it checks first for a launch storyboard, then for a launch xib, and then for launch images. If none of these are found, it defaults to showing a view controller with a white background.

• The code used for looking for a launch image is unchanged.
• The xib code is made slightly safer by checking that the class of the xib first object is a UIView.
• Class methods are used for all of these things as none of them need to refer to self.
• Grouped Splash Screen related methods under a #pragma mark and moved getBridge and getAppWindow to the top of the file.

In my tests, this works beautifully. I would be interested to know if it still works correctly for images and xibs.

Any feedback much appreciated.

[tmaly1980]

Does this have anything to do with why I'm getting a blank white screen with my fatal errors? Seems like something to do with storyboard files.

https://stackoverflow.com/questions/46206139/error-screen-with-white-background

[yogevbd]

Closes #2168, #2476