eframe: rename quit/exit to "close" (#1943)

Since https://github.com/emilk/egui/pull/1919 we can continue
the application after closing the native window. It therefore makes
more sense to call `frame.close()` to close the native window,
instead of `frame.quit()`.

examples/serial_windows/src/main.rs

@@ -46,7 +46,7 @@ impl eframe::App for MyApp {
         egui::CentralPanel::default().show(ctx, |ui| {
             if ui.button("Close").clicked() {
                 eprintln!("Pressed Close button");
-                frame.quit();
+                frame.close();
             }
         });
     }