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/mod.rs

@@ -51,4 +51,3 @@ pub(crate) use self::monitor::{
 use self::web_sys as backend;
 pub use self::window::{PlatformSpecificWindowAttributes, Window, WindowId};
 pub(crate) use crate::icon::NoIcon as PlatformIcon;
-pub(crate) use crate::platform_impl::Fullscreen;