api: make OwnedDisplayHandle wrap an opaque type

This will help in case we want to linger the event loop during drop
to prevent use after free in the consumers code.

src/platform_impl/web/mod.rs

@@ -38,7 +38,7 @@ pub(crate) use cursor::{
 };
 
 pub(crate) use self::event_loop::{
-    ActiveEventLoop, EventLoop, OwnedDisplayHandle, PlatformSpecificEventLoopAttributes,
+    ActiveEventLoop, EventLoop, PlatformSpecificEventLoopAttributes,
 };
 pub(crate) use self::keyboard::KeyEventExtra;
 pub(crate) use self::monitor::{