[iOS] Option to automatically generate icons and launch screens

[bruvzg]

• Adds export options to automatically generate missing iOS icons and launch screens from the project icon and boot splash.
• Adds validity and image size checks for user specified custom icons and launch screens (Xcode project build will fail if image sizes aren't correct).

[samgreen]

I've always wondered why Godot requires the launch images. Xcode will happily create one for us if one is not supplied, which makes the image generation work around seem a bit heavy handed.

Additionally since iOS 10 (the earliest iOS version Godot currently supports) we can simply provide a LaunchScreen.xib or LaunchScreen.storyboard file to allow Xcode to inflate that interface on run and then both the launch images and app icons would become unneccessary.

[akien-mga]

Thanks!

[Calinou]

It seems this pull request was never backported to the 3.x branch (despite this PR being merged one day before the master-3.x announcement).
Edit: Cherry-picked here: #49464

Also, why is generate_missing disabled by default? I'd expect it to be enabled by default to make the iOS export work better out of the box.