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()
 };
```

examples/custom_window_frame/src/main.rs

@@ -7,12 +7,12 @@ use eframe::egui::{self, ViewportCommand};
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        // Hide the OS-specific "chrome" around the window:
-        decorated: false,
-        // To have rounded corners we need transparency:
-        transparent: true,
-        min_window_size: Some(egui::vec2(400.0, 100.0)),
-        initial_window_size: Some(egui::vec2(400.0, 240.0)),
+        viewport: egui::ViewportBuilder::default()
+            .with_inner_size([400.0, 100.0])
+            .with_min_inner_size([400.0, 100.0])
+            .with_decorations(false) // Hide the OS-specific "chrome" around the window
+            .with_transparent(true), // To have rounded corners we need transparency
+
         ..Default::default()
     };
     eframe::run_native(