eframe web: Better panic handling (#2942)

* Refactor: remove extra store of events

* Remove unnecessary extra function

* Refactor: simplify event registering

* Store panic summary

* egui_demo_app: move web-part to own module

* index.html: await

* Properly unsubscribe from events on panic

* Better error handling

* Demo app html: hide the wasm canvas and show an error message on panic

* egui_demo_app: add panic button to test panic response on web

* fix typo

* Use a constructor to create WebHandle

* Refactor: less use of locks in the interfaces

* More consistent naming

crates/egui_demo_app/src/backend_panel.rs

@@ -108,11 +108,10 @@ impl BackendPanel {
             ui.ctx().set_debug_on_hover(debug_on_hover);
         }
 
-        ui.separator();
-
         #[cfg(target_arch = "wasm32")]
         #[cfg(feature = "web_screen-reader")]
         {
+            ui.separator();
             let mut screen_reader = ui.ctx().options(|o| o.screen_reader);
             ui.checkbox(&mut screen_reader, "🔈 Screen reader").on_hover_text("Experimental feature: checking this will turn on the screen reader on supported platforms");
             ui.ctx().options_mut(|o| o.screen_reader = screen_reader);
@@ -125,6 +124,15 @@ impl BackendPanel {
                 frame.close();
             }
         }
+
+        if cfg!(debug_assertions) && cfg!(target_arch = "wasm32") {
+            ui.separator();
+            // For testing panic handling on web:
+            #[allow(clippy::manual_assert)]
+            if ui.button("panic!()").clicked() {
+                panic!("intentional panic!");
+            }
+        }
     }
 
     fn integration_ui(&mut self, ui: &mut egui::Ui, frame: &mut eframe::Frame) {