Add Ui::close and Response::should_close (#5729)

This adds a generic way of telling containers to close from their child
`Ui`s.

* Part of #5727 
* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_demo_app/src/backend_panel.rs

@@ -333,7 +333,7 @@ fn integration_ui(ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
                     .send_viewport_cmd(egui::ViewportCommand::InnerSize(size));
                 ui.ctx()
                     .send_viewport_cmd(egui::ViewportCommand::Fullscreen(false));
-                ui.close_menu();
+                ui.close();
             }
         });
     }