Split AppBar Menu button into its own widget

Saves us the need to the GlobalKey as well.

lib/widgets/app_bar_menu_button.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/material.dart';
+import 'package:badges/badges.dart';
+
+import 'package:gitjournal/state_container.dart';
+
+class GJAppBarMenuButton extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final container = StateContainer.of(context);
+    final appState = container.appState;
+
+    bool shouldShowBadge =
+        !appState.remoteGitRepoConfigured && appState.hasJournalEntries;
+    var appBarMenuButton = BadgeIconButton(
+      key: const ValueKey("DrawerButton"),
+      icon: const Icon(Icons.menu),
+      itemCount: shouldShowBadge ? 1 : 0,
+      onPressed: () {
+        Scaffold.of(context).openDrawer();
+      },
+    );
+
+    return appBarMenuButton;
+  }
+}