chore: remove platform WindowId's

WindowId is a window _identifier_, and as such doesn't store anything
(unlike a _handle_). So we can safely make only be defined once, in the
core crate.

There are a few backends where we still use `into_raw` internally; I
consider these patterns discouraged, we should not be passing around
important state in the window id.

src/platform_impl/web/mod.rs

@@ -48,5 +48,5 @@ pub(crate) use self::monitor::{
     VideoModeHandle,
 };
 use self::web_sys as backend;
-pub use self::window::{PlatformSpecificWindowAttributes, Window, WindowId};
+pub use self::window::{PlatformSpecificWindowAttributes, Window};
 pub(crate) use crate::icon::NoIcon as PlatformIcon;