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/lib.rs

@@ -302,7 +302,7 @@ pub mod error;
 mod cursor;
 pub mod event;
 pub mod event_loop;
-mod icon;
+pub mod icon;
 pub mod keyboard;
 pub mod monitor;
 mod platform_impl;