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/svg/src/main.rs

@@ -0,0 +1,49 @@
+//! A good way of displaying an SVG image in egui.
+//!
+//! Requires the dependency `egui_extras` with the `svg` feature.
+
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() {
+    let options = eframe::NativeOptions {
+        initial_window_size: Some(egui::vec2(1000.0, 700.0)),
+        ..Default::default()
+    };
+    eframe::run_native(
+        "svg example",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+}
+
+struct MyApp {
+    svg_image: egui_extras::RetainedImage,
+}
+
+impl Default for MyApp {
+    fn default() -> Self {
+        Self {
+            svg_image: egui_extras::RetainedImage::from_svg_bytes(
+                "rustacean-flat-happy.svg",
+                include_bytes!("rustacean-flat-happy.svg"),
+            )
+            .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("SVG example");
+            ui.label("The SVG is rasterized and displayed as a texture.");
+
+            ui.separator();
+
+            let max_size = ui.available_size();
+            self.svg_image.show_max_size(ui, max_size);
+        });
+    }
+}