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/orbital/mod.rs

@@ -5,7 +5,7 @@ use std::{fmt, str};
 
 use smol_str::SmolStr;
 
-pub(crate) use self::event_loop::{ActiveEventLoop, EventLoop, EventLoopProxy, OwnedDisplayHandle};
+pub(crate) use self::event_loop::{ActiveEventLoop, EventLoop, OwnedDisplayHandle};
 use crate::dpi::{PhysicalPosition, PhysicalSize};
 use crate::keyboard::Key;
 mod event_loop;