feat: allow on_app_exit to override closing the application

src/app/cosmic.rs

@@ -449,8 +449,10 @@ impl<T: Application> Cosmic<T> {
             }
 
             Message::Close => {
-                self.app.on_app_exit();
-                return self.close();
+                return match self.app.on_app_exit() {
+                    Some(message) => self.app.update(message),
+                    None => self.close(),
+                };
             }
             Message::SystemThemeModeChange(keys, mode) => {
                 let mut cmds = vec![self.app.system_theme_mode_update(&keys, &mode)];

src/app/mod.rs

@@ -494,8 +494,10 @@ where
         None
     }
 
-    /// Called before closing the application.
-    fn on_app_exit(&mut self) {}
+    /// Called before closing the application. Returning a message will override closing windows.
+    fn on_app_exit(&mut self) -> Option<Self::Message> {
+        None
+    }
 
     /// Called when a window requests to be closed.
     fn on_close_requested(&self, id: window::Id) -> Option<Self::Message> {