Photo search: add Quit menu item (#998)

Adds a Quit menu item to exit the app.

desktop_photo_search/fluent_ui/lib/main.dart

@@ -6,6 +6,7 @@ import 'dart:io';
 
 import 'package:fluent_ui/fluent_ui.dart';
 import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
 import 'package:logging/logging.dart';
 import 'package:menubar/menubar.dart' as menubar;
 import 'package:provider/provider.dart';
@@ -85,6 +86,13 @@ class UnsplashHomePage extends StatelessWidget {
             );
           },
         ),
+        if (!Platform.isMacOS)
+          menubar.MenuItem(
+            label: 'Quit',
+            onClicked: () {
+              SystemNavigator.pop();
+            },
+          ),
       ]),
       menubar.Submenu(label: 'About', children: [
         menubar.MenuItem(