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/apple/appkit/mod.rs

@@ -21,4 +21,3 @@ pub(crate) use self::event_loop::{
 pub(crate) use self::monitor::MonitorHandle;
 pub(crate) use self::window::Window;
 pub(crate) use self::window_delegate::PlatformSpecificWindowAttributes;
-pub(crate) use crate::icon::NoIcon as PlatformIcon;