iOS Abstract Out the UIView type from winit

[mtak-]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior

There are a lot of changes to iOS in this pull request.

• The UIApplication setjmp hacks now happen in Window::new. This allows PlatformSpecificWindowBuilderAttributes to customize the specific UIView class.
• The biggest breaking change here is that winit no longer sets up a GLKView. Putting responsibility on other libraries like glutin and vulkano-win to specify the exact UIView subclass.
• EventsLoop now explicitly checks that it's created on the main thread. AFAICT this was required before, but it was just UB to create it anywhere else.
• Only one Window can ever be created.
• EventsLoop cannot be polled until a Window has been created - in order for the CFRunLoopRunInMode calls to work.

I have been testing it with glutin. mtak-/glutin@1a3066f

Let me know what needs to be changed/etc.

[francesca64]

This looks fantastic, thanks!