iOS 13 support

[defagos]

iOS 13 support should be quite straightforward. Testing the demo I still spotted a few issues:

• UIStepper bindings do not work anymore.

I also found other issues not related to iOS 13:

• Parallax demos are broken.
• Linking with CoconutKit and MAKVONotificationCenter reveals category method conflicts (newly reported by Xcode 11).
• Using the cursor resizing sliders make the demo crash.