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/linux/mod.rs

@@ -13,7 +13,6 @@ use crate::application::ApplicationHandler;
 use crate::dpi::Size;
 use crate::error::{EventLoopError, NotSupportedError};
 use crate::event_loop::ActiveEventLoop;
-pub(crate) use crate::icon::RgbaIcon as PlatformIcon;
 use crate::platform::pump_events::PumpStatus;
 #[cfg(x11_platform)]
 use crate::platform::x11::WindowType as XWindowType;