Update example app on Flutter plugin (#220)

* Update example app on Flutter plugin

* Expose `empty_wallet_cache` through Dart bindings (#224)

packages/flutter/example/lib/routes/home/widgets/mnemonics_dialog.dart

@@ -0,0 +1,49 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+class MnemonicsDialog extends StatelessWidget {
+  final List<String> mnemonics;
+
+  MnemonicsDialog({super.key, required this.mnemonics});
+
+  final textFieldControllers = List<TextEditingController>.generate(12, (_) => TextEditingController());
+
+  @override
+  Widget build(BuildContext context) {
+    return AlertDialog(
+      title: const Text("Mnemonics"),
+      content: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          GridView.builder(
+            shrinkWrap: true,
+            gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
+              maxCrossAxisExtent: MediaQuery.of(context).size.width / 2,
+              childAspectRatio: 2,
+              crossAxisSpacing: 8,
+              mainAxisSpacing: 8,
+            ),
+            itemCount: mnemonics.length,
+            itemBuilder: (BuildContext context, int index) {
+              textFieldControllers[index].text = mnemonics.elementAt(index);
+              return TextField(
+                readOnly: true,
+                controller: textFieldControllers[index],
+                decoration: InputDecoration(labelText: "${index + 1}", border: InputBorder.none),
+                inputFormatters: [FilteringTextInputFormatter.deny(RegExp(r"\s\b|\b\s"))],
+              );
+            },
+          )
+        ],
+      ),
+      actions: [
+        TextButton(
+          child: const Text("CLOSE"),
+          onPressed: () {
+            Navigator.of(context).pop();
+          },
+        ),
+      ],
+    );
+  }
+}