iOS packaging: Add Info.plist, launch screen and app icon

[fwcd]

This adds packaging assets used by the iOS app:

• A borderless rectangular app icon, based on Add a Big Sur-style rounded rectangle icon on macOS #4545
  
  • Like for the macOS icon, a script is provided to regenerate the PNG (which is actually used) from the SVG
• An Info.plist that e.g. also provides the permission to view Mixxx's sandbox directories from the iOS files app
• A launch screen that displays the Mixxx logo (which is included as PDF, the format iOS expects) before any UI is loaded:
  

[daschuer]

Did you consider the option to not add extra icons for iOS, but link existing? This will reduce some maintenance in future.

[fwcd]

Yeah, the problem is that iOS needs a borderless icon (as opposed to macOS), therefore we need a separate one. Technically we could use the bordered one, but the App Store generally disallows transparent icons and if iOS changes the corner radius again (they did that back in iOS 7 IIRC), the icon would look strange again.

[JoergAtGithub]

LGTM! Thank you!