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/file_dialog/src/main.rs

@@ -0,0 +1,103 @@
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
+
+use eframe::egui;
+
+fn main() {
+    let options = eframe::NativeOptions {
+        drag_and_drop_support: true,
+        ..Default::default()
+    };
+    eframe::run_native(
+        "Native file dialogs and drag-and-drop files",
+        options,
+        Box::new(|_cc| Box::new(MyApp::default())),
+    );
+}
+
+#[derive(Default)]
+struct MyApp {
+    dropped_files: Vec<egui::DroppedFile>,
+    picked_path: Option<String>,
+}
+
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+        egui::CentralPanel::default().show(ctx, |ui| {
+            ui.label("Drag-and-drop files onto the window!");
+
+            if ui.button("Open file…").clicked() {
+                if let Some(path) = rfd::FileDialog::new().pick_file() {
+                    self.picked_path = Some(path.display().to_string());
+                }
+            }
+
+            if let Some(picked_path) = &self.picked_path {
+                ui.horizontal(|ui| {
+                    ui.label("Picked file:");
+                    ui.monospace(picked_path);
+                });
+            }
+
+            // Show dropped files (if any):
+            if !self.dropped_files.is_empty() {
+                ui.group(|ui| {
+                    ui.label("Dropped files:");
+
+                    for file in &self.dropped_files {
+                        let mut info = if let Some(path) = &file.path {
+                            path.display().to_string()
+                        } else if !file.name.is_empty() {
+                            file.name.clone()
+                        } else {
+                            "???".to_owned()
+                        };
+                        if let Some(bytes) = &file.bytes {
+                            info += &format!(" ({} bytes)", bytes.len());
+                        }
+                        ui.label(info);
+                    }
+                });
+            }
+        });
+
+        self.detect_files_being_dropped(ctx);
+    }
+}
+
+impl MyApp {
+    fn detect_files_being_dropped(&mut self, ctx: &egui::Context) {
+        use egui::*;
+
+        // Preview hovering files:
+        if !ctx.input().raw.hovered_files.is_empty() {
+            let mut text = "Dropping files:\n".to_owned();
+            for file in &ctx.input().raw.hovered_files {
+                if let Some(path) = &file.path {
+                    text += &format!("\n{}", path.display());
+                } else if !file.mime.is_empty() {
+                    text += &format!("\n{}", file.mime);
+                } else {
+                    text += "\n???";
+                }
+            }
+
+            let painter =
+                ctx.layer_painter(LayerId::new(Order::Foreground, Id::new("file_drop_target")));
+
+            let screen_rect = ctx.input().screen_rect();
+            painter.rect_filled(screen_rect, 0.0, Color32::from_black_alpha(192));
+            painter.text(
+                screen_rect.center(),
+                Align2::CENTER_CENTER,
+                text,
+                TextStyle::Heading.resolve(&ctx.style()),
+                Color32::WHITE,
+            );
+        }
+
+        // Collect dropped files:
+        if !ctx.input().raw.dropped_files.is_empty() {
+            self.dropped_files = ctx.input().raw.dropped_files.clone();
+        }
+    }
+}