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/retained_image/src/main.rs

@@ -0,0 +1,53 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+use egui_extras::RetainedImage;
+
+fn main() {
+    let options = eframe::NativeOptions {
+        initial_window_size: Some(egui::vec2(500.0, 900.0)),
+        ..Default::default()
+    };
+    eframe::run_native(
+        "Show an image with eframe/egui",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+}
+
+struct MyApp {
+    image: RetainedImage,
+}
+
+impl Default for MyApp {
+    fn default() -> Self {
+        Self {
+            image: RetainedImage::from_image_bytes(
+                "rust-logo-256x256.png",
+                include_bytes!("rust-logo-256x256.png"),
+            )
+            .unwrap(),
+        }
+    }
+}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.heading("This is an image:");
+            self.image.show(ui);
+
+            ui.heading("This is a rotated image:");
+            ui.add(
+                egui::Image::new(self.image.texture_id(ctx), self.image.size_vec2())
+                    .rotate(45.0_f32.to_radians(), egui::Vec2::splat(0.5)),
+            );
+
+            ui.heading("This is an image you can click:");
+            ui.add(egui::ImageButton::new(
+                self.image.texture_id(ctx),
+                self.image.size_vec2(),
+            ));
+        });
+    }
+}

examples/retained_image/src/rust-logo-license.txt

@@ -0,0 +1 @@
+Rust logo by Mozilla, from https://github.com/rust-lang/rust-artwork