Move examples out of eframe/examples into examples/ (#1486)

* Move examples out of eframe/examples into examples/

Give each example a `Cargo.toml` and `src/main.rs`.
This makes it easier for people to use as templates.

* Update README.md with more deps needed on vanilla Ubuntu
* Install libgtk-3-dev on CI, hoping that will fix something

examples/confirm_exit/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "confirm_exit"
+version = "0.1.0"
+authors = ["Emil Ernerfeldt <emil.ernerfeldt@gmail.com>"]
+license = "MIT OR Apache-2.0"
+edition = "2021"
+rust-version = "1.60"
+publish = false
+
+
+[dependencies]
+eframe = { path = "../../eframe" }

examples/confirm_exit/README.md

@@ -0,0 +1,3 @@
+```sh
+cargo run -p confirm_exit
+```

examples/confirm_exit/src/main.rs

@@ -0,0 +1,49 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() {
+    let options = eframe::NativeOptions::default();
+    eframe::run_native(
+        "Confirm exit",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+}
+
+#[derive(Default)]
+struct MyApp {
+    can_exit: bool,
+    is_exiting: bool,
+}
+
+impl eframe::App for MyApp {
+    fn on_exit_event(&mut self) -> bool {
+        self.is_exiting = true;
+        self.can_exit
+    }
+
+    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("Try to close the window");
+        });
+
+        if self.is_exiting {
+            egui::Window::new("Do you want to quit?")
+                .collapsible(false)
+                .resizable(false)
+                .show(ctx, |ui| {
+                    ui.horizontal(|ui| {
+                        if ui.button("Not yet").clicked() {
+                            self.is_exiting = false;
+                        }
+
+                        if ui.button("Yes!").clicked() {
+                            self.can_exit = true;
+                            frame.quit();
+                        }
+                    });
+                });
+        }
+    }
+}