eframe: add a simplified native-only API for simple native apps (#2453)

* eframe: add a simplified native-only API for simple native apps

* build-fix

* Fix doc-test

examples/hello_world_simple/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "hello_world_simple"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.65"
+publish = false
+
+
+[dependencies]
+eframe = { path = "../../crates/eframe", features = [
+    "__screenshot", # __screenshot is so we can dump a ascreenshot using EFRAME_SCREENSHOT_TO
+] }
+tracing-subscriber = "0.3"

examples/hello_world_simple/README.md

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+Example showing some UI controls like `Label`, `TextEdit`, `Slider`, `Button`.
+
+```sh
+cargo run -p hello_world
+```
+
+![](screenshot.png)

examples/hello_world_simple/src/main.rs

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+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() -> Result<(), eframe::Error> {
+    // Log to stdout (if you run with `RUST_LOG=debug`).
+    tracing_subscriber::fmt::init();
+
+    let options = eframe::NativeOptions {
+        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        ..Default::default()
+    };
+
+    // Our application state:
+    let mut name = "Arthur".to_owned();
+    let mut age = 42;
+
+    eframe::run_simple_native("My egui App", options, move |ctx, _frame| {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("My egui Application");
+            ui.horizontal(|ui| {
+                let name_label = ui.label("Your name: ");
+                ui.text_edit_singleline(&mut name)
+                    .labelled_by(name_label.id);
+            });
+            ui.add(egui::Slider::new(&mut age, 0..=120).text("age"));
+            if ui.button("Click each year").clicked() {
+                age += 1;
+            }
+            ui.label(format!("Hello '{}', age {}", name, age));
+        });
+    })
+}