api: convert Window to dyn Window

This should allow us to make future split of backends much easier.
The `Box<dyn Window>` is a _temporary_ solution, which will be
removed with the future updates when we decide on how the Window
should be stored.

src/platform_impl/web/monitor.rs

@@ -29,6 +29,7 @@ use super::main_thread::MainThreadMarker;
 use super::r#async::{Dispatcher, Notified, Notifier};
 use super::web_sys::{Engine, EventListenerHandle};
 use crate::dpi::{PhysicalPosition, PhysicalSize};
+use crate::monitor::MonitorHandle as RootMonitorHandle;
 use crate::platform::web::{
     MonitorPermissionError, Orientation, OrientationData, OrientationLock, OrientationLockError,
 };
@@ -188,6 +189,12 @@ impl PartialOrd for MonitorHandle {
     }
 }
 
+impl From<MonitorHandle> for RootMonitorHandle {
+    fn from(inner: MonitorHandle) -> Self {
+        RootMonitorHandle { inner }
+    }
+}
+
 #[derive(Debug)]
 pub enum OrientationLockFuture {
     Future(Notified<Result<(), OrientationLockError>>),