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/web/mod.rs

@@ -42,4 +42,3 @@ pub(crate) use self::monitor::{
 };
 use self::web_sys as backend;
 pub use self::window::{PlatformSpecificWindowAttributes, Window};
-pub(crate) use crate::icon::NoIcon as PlatformIcon;