Optimize iOS full-screen back gesture implementation (#643)

* Optimize iOS full-screen back gesture implementation

- Fix #613 and #617

* Fix setting page

lib/pages/comic_source_page.dart

@@ -200,7 +200,6 @@ class _BodyState extends State<_Body> {
         await ComicSourceManager().reload();
         setState(() {});
       }),
-      iosFullScreenGesture: false,
     );
   }