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/main.rs

@@ -10,12 +10,13 @@ fn main() {
     // Log to stdout (if you run with `RUST_LOG=debug`).
     tracing_subscriber::fmt::init();
 
-    let app = egui_demo_lib::WrapApp::default();
     let options = eframe::NativeOptions {
         // Let's show off that we support transparent windows
         transparent: true,
         drag_and_drop_support: true,
         ..Default::default()
     };
-    eframe::run_native(Box::new(app), options);
+    eframe::run_native("egui demo app", options, |cc| {
+        Box::new(egui_demo_lib::WrapApp::new(cc))
+    });
 }