Xchaha20 poly1305 integration (#569)

* Integration of xchacha20-poly1305

* Remove force unwrap from _exportPreferencesJSON

* Deprecate v1 export

* Fix for open backup screen after auth

lib/src/screens/backup/backup_page.dart

@@ -1,10 +1,10 @@
 import 'dart:io';
 import 'package:cake_wallet/palette.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/cupertino.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_mobx/flutter_mobx.dart';
-// import 'package:esys_flutter_share/esys_flutter_share.dart';
+import 'package:share_plus/share_plus.dart';
+import 'package:cross_file/cross_file.dart';
 import 'package:cake_wallet/utils/show_bar.dart';
 import 'package:cake_wallet/routes.dart';
 import 'package:cake_wallet/generated/i18n.dart';
@@ -103,12 +103,14 @@ class BackupPage extends BasePage {
                 Navigator.of(dialogContext).pop();
                 final backup = await backupViewModelBase.exportBackup();
 
+                if (backup == null) {
+                  return;
+                }
+
                 if (Platform.isAndroid) {
-                  onExportAndroid(context, backup!);
+                  onExportAndroid(context, backup);
                 } else {
-                  // FIX-ME: Share esys_flutter_share.dart
-                  // await Share.file(S.of(context).backup_file, backup.name,
-                  //     backup.content, 'application/*');
+                  await share(backup);
                 }
               },
               actionLeftButton: () => Navigator.of(dialogContext).pop());
@@ -136,12 +138,20 @@ class BackupPage extends BasePage {
                     backup.name, backup.content);
                 Navigator.of(dialogContext).pop();
               },
-              actionLeftButton: () {
+              actionLeftButton: () async {
                 Navigator.of(dialogContext).pop();
-                // FIX-ME: Share esys_flutter_share.dart
-                // Share.file(S.of(context).backup_file, backup.name,
-                //     backup.content, 'application/*');
+                await share(backup);
               });
         });
   }
+
+  Future<void> share(BackupExportFile backup) async {
+    const mimeType = 'application/*';
+    final path = await backupViewModelBase.saveBackupFileLocally(backup);
+    await Share.shareXFiles(<XFile>[XFile(
+        path,
+        name: backup.name,
+        mimeType: mimeType)]);
+    await backupViewModelBase.removeBackupFileLocally(backup);
+  }
 }

lib/src/screens/dashboard/wallet_menu.dart

@@ -51,6 +51,21 @@ class WalletMenu {
           image: Image.asset('assets/images/open_book_menu.png',
               height: 16, width: 16),
 	  handler: () => Navigator.of(context).pushNamed(Routes.addressBook)),
+      WalletMenuItem(
+        title: S.current.backup,
+        image: Image.asset('assets/images/restore_wallet.png',
+          height: 16,
+          width: 16,
+          color: Palette.darkBlue),
+        handler: () {
+          Navigator.of(context).pushNamed(
+            Routes.auth,
+            arguments: (bool isAuthenticatedSuccessfully, AuthPageState auth) {
+              if (isAuthenticatedSuccessfully) {
+                auth.close(route:Routes.backup);
+              }
+            });
+        }),
       WalletMenuItem(
           title: S.current.settings_title,
           image: Image.asset('assets/images/settings_menu.png',