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/mod.rs

@@ -277,14 +277,6 @@ pub enum EventLoop {
     X(x11::EventLoop),
 }
 
-#[derive(Clone)]
-pub enum EventLoopProxy {
-    #[cfg(x11_platform)]
-    X(x11::EventLoopProxy),
-    #[cfg(wayland_platform)]
-    Wayland(wayland::EventLoopProxy),
-}
-
 impl EventLoop {
     pub(crate) fn new(
         attributes: &PlatformSpecificEventLoopAttributes,
@@ -404,12 +396,6 @@ impl AsRawFd for EventLoop {
     }
 }
 
-impl EventLoopProxy {
-    pub fn wake_up(&self) {
-        x11_or_wayland!(match self; EventLoopProxy(proxy) => proxy.wake_up())
-    }
-}
-
 #[derive(Clone)]
 #[allow(dead_code)]
 pub(crate) enum OwnedDisplayHandle {