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/wrap_app.rs

@@ -384,12 +384,12 @@ impl WrapApp {
                 .clicked()
             {
                 ui.ctx().memory_mut(|mem| *mem = Default::default());
-                ui.close_menu();
+                ui.close();
             }
 
             if ui.button("Reset everything").clicked() {
                 *cmd = Command::ResetEverything;
-                ui.close_menu();
+                ui.close();
             }
         });
     }