Use egui::ViewportBuilder in eframe::NativeOptions (#3572)

* Part of https://github.com/emilk/egui/issues/3556

This PR replaces a bunch of options in `eframe::NativeOptions` with
`egui::ViewportBuilder`. For instance:

``` diff
 let options = eframe::NativeOptions {
-    initial_window_size: Some(egui::vec2(320.0, 240.0)),
-    drag_and_drop_support: true,
+    viewport: egui::ViewportBuilder::default()
+        .with_inner_size([320.0, 240.0])
+        .with_drag_and_drop(true),
     centered: true,
     ..Default::default()
 };
```

crates/eframe/src/lib.rs

@@ -166,10 +166,19 @@ mod native;
 #[cfg(feature = "persistence")]
 pub use native::file_storage::storage_dir;
 
+#[cfg(not(target_arch = "wasm32"))]
+pub mod icon_data;
+
 /// This is how you start a native (desktop) app.
 ///
-/// The first argument is name of your app, used for the title bar of the native window
-/// and the save location of persistence (see [`App::save`]).
+/// The first argument is name of your app, which is a an identifier
+/// used for the save location of persistence (see [`App::save`]).
+/// It is also used as the application id on wayland.
+/// If you set no title on the viewport, the app id will be used
+/// as the title.
+///
+/// For details about application ID conventions, see the
+/// [Desktop Entry Spec](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#desktop-file-id)
 ///
 /// Call from `fn main` like this:
 /// ``` no_run
@@ -209,7 +218,7 @@ pub use native::file_storage::storage_dir;
 #[allow(clippy::needless_pass_by_value)]
 pub fn run_native(
     app_name: &str,
-    native_options: NativeOptions,
+    mut native_options: NativeOptions,
     app_creator: AppCreator,
 ) -> Result<()> {
     let renderer = native_options.renderer;
@@ -220,6 +229,10 @@ pub fn run_native(
         "EFRAME_SCREENSHOT_TO found without compiling with the '__screenshot' feature"
     );
 
+    if native_options.viewport.title.is_none() {
+        native_options.viewport.title = Some(app_name.to_owned());
+    }
+
     match renderer {
         #[cfg(feature = "glow")]
         Renderer::Glow => {