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/linux/wayland/event_loop/proxy.rs

@@ -1,19 +1,30 @@
 //! An event loop proxy.
 
+use std::sync::Arc;
+
 use sctk::reexports::calloop::ping::Ping;
 
+use crate::event_loop::{EventLoopProxy as CoreEventLoopProxy, EventLoopProxyProvider};
+
 /// A handle that can be sent across the threads and used to wake up the `EventLoop`.
-#[derive(Clone)]
 pub struct EventLoopProxy {
     ping: Ping,
 }
 
+impl EventLoopProxyProvider for EventLoopProxy {
+    fn wake_up(&self) {
+        self.ping.ping();
+    }
+}
+
 impl EventLoopProxy {
     pub fn new(ping: Ping) -> Self {
         Self { ping }
     }
+}
 
-    pub fn wake_up(&self) {
-        self.ping.ping();
+impl From<EventLoopProxy> for CoreEventLoopProxy {
+    fn from(value: EventLoopProxy) -> Self {
+        CoreEventLoopProxy::new(Arc::new(value))
     }
 }