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/event_loop/mod.rs

@@ -1,4 +1,4 @@
-use super::{backend, window, HasMonitorPermissionFuture, MonitorPermissionFuture};
+use super::{backend, HasMonitorPermissionFuture, MonitorPermissionFuture};
 use crate::application::ApplicationHandler;
 use crate::error::{EventLoopError, NotSupportedError};
 use crate::event::Event;