eframe app creation refactor (#1363)

* Change how eframe apps are created
* eframe: re-export epi::* so users don't need to care about what epi is

eframe/examples/file_dialog.rs

@@ -1,6 +1,18 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 
-use eframe::{egui, epi};
+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,
+        |_cc| Box::new(MyApp::default()),
+    );
+}
 
 #[derive(Default)]
 struct MyApp {
@@ -8,12 +20,8 @@ struct MyApp {
     picked_path: Option<String>,
 }
 
-impl epi::App for MyApp {
-    fn name(&self) -> &str {
-        "Native file dialogs and drag-and-drop files"
-    }
-
-    fn update(&mut self, ctx: &egui::Context, _frame: &epi::Frame) {
+impl eframe::App for MyApp {
+    fn update(&mut self, ctx: &egui::Context, _frame: &eframe::Frame) {
         egui::CentralPanel::default().show(ctx, |ui| {
             ui.label("Drag-and-drop files onto the window!");
 
@@ -93,11 +101,3 @@ impl MyApp {
         }
     }
 }
-
-fn main() {
-    let options = eframe::NativeOptions {
-        drag_and_drop_support: true,
-        ..Default::default()
-    };
-    eframe::run_native(Box::new(MyApp::default()), options);
-}