Wrap the proxy inside the Neovim handler with an Arc and Mutex

The handler need to be send + sync + clone, and that's not the case on
all platforms.

src/bridge/handler.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+use std::sync::Mutex;
 use async_trait::async_trait;
 use log::trace;
 use nvim_rs::{Handler, Neovim};
@@ -19,12 +21,13 @@ use crate::{
 
 #[derive(Clone)]
 pub struct NeovimHandler {
-    proxy: EventLoopProxy<UserEvent>,
+    // The EventLoopProxy is not sync on all platforms, so wrap it in mutex
+    proxy: Arc<Mutex<EventLoopProxy<UserEvent>>>,
 }
 
 impl NeovimHandler {
     pub fn new(proxy: EventLoopProxy<UserEvent>) -> Self {
-        Self { proxy }
+        Self { proxy: Arc::new(Mutex::new(proxy)) }
     }
 }
 
@@ -102,7 +105,7 @@ impl Handler for NeovimHandler {
             }
             "neovide.focus_window" => {
                 let _ = self
-                    .proxy
+                    .proxy.lock().unwrap()
                     .send_event(UserEvent::WindowCommand(WindowCommand::FocusWindow));
             }
             _ => {}