api: make EventLoopProxy wrap an opaque type

The proxy is intended to be Clone, thus use `Arc` for it internally and
don't require backends for it to be `Clone`. Use `EventLoopProxyProvider`
to hide the backend's proxy implementation details.

Co-authored-by: daxpedda <daxpedda@gmail.com>

src/platform_impl/web/mod.rs

@@ -38,8 +38,7 @@ pub(crate) use cursor::{
 };
 
 pub(crate) use self::event_loop::{
-    ActiveEventLoop, EventLoop, EventLoopProxy, OwnedDisplayHandle,
-    PlatformSpecificEventLoopAttributes,
+    ActiveEventLoop, EventLoop, OwnedDisplayHandle, PlatformSpecificEventLoopAttributes,
 };
 pub(crate) use self::keyboard::KeyEventExtra;
 pub(crate) use self::monitor::{