iOS: generate storyboard splashscreens (mandatory for iPhone X)

[znegva]

The preferred way to provide splashscreens for iOS seems to be to use so called storyboard-images, see Cordova Plugin Splashscreen for details.

If no correct splash is provided for an App running on iPhone X it will not be shown in fullscreen, see here.

I adjusted the generated splashscreens for iOS and the README on how to include them in config.xml. I used the naming conventions from Cordova Plugin Splashscreen.