Fix panic when deploying emoji picker (character palette)

The panic was caused by Cocoa synchronously invoking the `selected_text_range`
method on the registered input handler while we already had a borrow of the
app.

This commit fixes this issue by showing the character palette on the next tick
of the loop (we've had this problem in other spots too and used the same technique).

crates/gpui2/src/platform/mac/window.rs

@@ -886,11 +886,16 @@ impl PlatformWindow for MacWindow {
     }
 
     fn show_character_palette(&self) {
+        let this = self.0.lock();
+        let window = this.native_window;
+        this.executor
+            .spawn(async move {
                 unsafe {
                     let app = NSApplication::sharedApplication(nil);
-            let window = self.0.lock().native_window;
                     let _: () = msg_send![app, orderFrontCharacterPalette: window];
                 }
+            })
+            .detach();
     }
 
     fn minimize(&self) {