Add a 'Restore Purchase' button

Instead of doing it automatically,

Fixes #211

assets/langs/en.yaml

@@ -221,3 +221,4 @@ purchase_screen:
 Paying a monthly subscription helps you support development and gives you access to
 a number of "Pro" only features. We operate on a "pay what you want model". No matter what you pay
 you will get access to all the Pro features.'
+  restore: Restore Purchase

lib/screens/purchase_screen.dart

@@ -23,7 +23,6 @@ class PurchaseScreen extends StatelessWidget {
   }
 
   Widget buildBody(BuildContext context) {
-
     Widget w = Column(
       children: <Widget>[
         Text(
@@ -43,6 +42,8 @@ class PurchaseScreen extends StatelessWidget {
         ),
         const SizedBox(height: 64.0),
         PurchaseWidget(),
+        const SizedBox(height: 32.0),
+        RestorePurchaseButton(),
       ],
       mainAxisAlignment: MainAxisAlignment.start,
       mainAxisSize: MainAxisSize.min,

lib/widgets/purchase_widget.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:easy_localization/easy_localization.dart';
 import 'package:flutter/material.dart';
 
 import 'package:in_app_purchase/in_app_purchase.dart';
@@ -74,13 +75,6 @@ class _PurchaseWidgetState extends State<PurchaseWidget> {
   }
 
   Future<void> initPlatformState() async {
-    // In parallel check if the purchase has been made
-    InAppPurchases.confirmProPurchase().then((void _) {
-      if (Settings.instance.proMode) {
-        Navigator.of(context).pop();
-      }
-    });
-
     final iapCon = InAppPurchaseConnection.instance;
 
     final bool available = await iapCon.isAvailable();
@@ -335,3 +329,30 @@ class PurchaseFailedDialog extends StatelessWidget {
     );
   }
 }
+
+class RestorePurchaseButton extends StatefulWidget {
+  @override
+  _RestorePurchaseButtonState createState() => _RestorePurchaseButtonState();
+}
+
+class _RestorePurchaseButtonState extends State<RestorePurchaseButton> {
+  bool computing = false;
+
+  @override
+  Widget build(BuildContext context) {
+    var text = computing ? '...' : tr('purchase_screen.restore');
+
+    return OutlineButton(
+      child: Text(text),
+      onPressed: () async {
+        setState(() {
+          computing = true;
+        });
+        await InAppPurchases.confirmProPurchase();
+        if (Settings.instance.proMode) {
+          Navigator.of(context).pop();
+        }
+      },
+    );
+  }
+}