add remote notification registration app delegates

[extrawurst]

I need some input here :)

I am not sure about the way to bubble up the events. For now i added a new one entirely but that feels wrong. do you think it should rather be a DeviceEvent cause it is for sure not a window event, right?

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[kchibisov]

branch must be opened against master.

For events see #3474

[madsmtm]

Instead of exposing this through Winit, #3758 is going to allow register a custom UIApplicationDelegate outside of Winit using objc2::declare_class!, Objective-C, Swift, or similar.