Add rate app link

app/lib/widget/settings.dart

@@ -138,6 +138,13 @@ class _SettingsState extends State<Settings> {
                     }
                   },
                 ),
+                ListTile(
+                  title: const Text("Rate on Google Play"),
+                  onTap: () {
+                    launch(
+                        "https://play.google.com/store/apps/details?id=com.nkming.nc_photos.paid");
+                  },
+                ),
                 ListTile(
                   title: Text(L10n.global().settingsSourceCodeTitle),
                   onTap: () {