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/android/mod.rs

@@ -20,7 +20,6 @@ use crate::event_loop::{
     EventLoopProxy as CoreEventLoopProxy, EventLoopProxyProvider,
     OwnedDisplayHandle as CoreOwnedDisplayHandle,
 };
-pub(crate) use crate::icon::NoIcon as PlatformIcon;
 use crate::monitor::{Fullscreen, MonitorHandle as CoreMonitorHandle};
 use crate::platform::pump_events::PumpStatus;
 use crate::window::{