Use egui::ViewportBuilder in eframe::NativeOptions

[emilk]

• Part of Tracking issue: Misc improvements to multi-viewports #3556

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

 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()
 };