linux: create a hidden window inside the platform

It allows us to receive messages from the dispatcher,
which breaks us out of waiting and lets us execute
main thread runnables as a part of the main loop.

crates/gpui/src/platform/linux/window.rs

@@ -237,6 +237,7 @@ impl LinuxWindowState {
         if let Some(fun) = self.callbacks.lock().close.take() {
             fun();
         }
+        self.xcb_connection.flush().unwrap();
     }
 
     pub fn expose(&self) {