Remove WindowBuilder in favour of WindowAttributes

[madsmtm]

Blocked on #3318.

Rough draft, the plan is to move towards:

let attrs = WindowAttributes::new().with_title("MyTitle");
let window = event_loop.create_window(attrs).unwrap();

I think in this case it makes sense to remove the builder, because it really is just attributes; but in a case like EventLoopBuilder, I think it still makes sense to keep that as a builder.

• 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
• 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]

I think I have it ported on winit-next the way you suggested, however there's an issue with extensibility of it in the future. Like with dynamic backend we can't really account for everything ahead of time, so I was thinking that maybe we should create a trait with defaults?

But in general having the same type is fine by me. it's called a builder because of the pattern with the build, but really it's just a bunch of attributes and a method to set them.

[daxpedda]

Closing in favor #3447.