eframe app creation refactor (#1363)

* Change how eframe apps are created
* eframe: re-export epi::* so users don't need to care about what epi is

egui_demo_app/src/lib.rs

@@ -19,6 +19,5 @@ pub fn start(canvas_id: &str) -> Result<(), wasm_bindgen::JsValue> {
     // Redirect tracing to console.log and friends:
     tracing_wasm::set_as_global_default();
 
-    let app = egui_demo_lib::WrapApp::default();
-    eframe::start_web(canvas_id, Box::new(app))
+    eframe::start_web(canvas_id, |cc| Box::new(egui_demo_lib::WrapApp::new(cc)))
 }