Add an eframe example app

eframe/examples/hello_world.rs

@@ -0,0 +1,45 @@
+use eframe::{egui, epi};
+
+struct MyApp {
+    name: String,
+    age: u32,
+}
+
+impl Default for MyApp {
+    fn default() -> Self {
+        Self {
+            name: "Arthur".to_owned(),
+            age: 42,
+        }
+    }
+}
+
+impl epi::App for MyApp {
+    fn name(&self) -> &str {
+        "My Egui App"
+    }
+
+    fn update(&mut self, ctx: &egui::CtxRef, frame: &mut epi::Frame<'_>) {
+        let Self { name, age } = self;
+
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("My Egui Application");
+            ui.horizontal(|ui| {
+                ui.label("Your name: ");
+                ui.text_edit_singleline(name);
+            });
+            ui.add(egui::Slider::u32(age, 0..=120).text("age"));
+            if ui.button("Click each year").clicked {
+                *age += 1;
+            }
+            ui.label(format!("Hello '{}', age {}", name, age));
+        });
+
+        // Resize the native window to be just the size we need it to be:
+        frame.set_window_size(ctx.used_size());
+    }
+}
+
+fn main() {
+    eframe::run_native(Box::new(MyApp::default()));
+}