icon: refactor Icon to be dyn

Same as for `CustomCursor`. However, the API uses `dyn` stuff only
because of `Windows` backend at the time of writing, generally, platforms
should just have a separate method that deals with all of that, and e.g.
top-level winit only guarantees `Rgba`.

src/platform_impl/orbital/mod.rs

@@ -4,7 +4,6 @@ use std::{fmt, str};
 
 pub(crate) use self::event_loop::{ActiveEventLoop, EventLoop};
 pub use self::window::Window;
-pub(crate) use crate::icon::NoIcon as PlatformIcon;
 
 mod event_loop;
 mod window;