[#8] Add auto update checker

Elwig/Dialogs/UpdateDialog.xaml.cs

@@ -0,0 +1,47 @@
+using Elwig.Helpers;
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Net.Http;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace Elwig.Dialogs {
+    public partial class UpdateDialog : Window {
+
+        public string Version { get; private set; }
+        public string Url { get; private set; }
+
+        public UpdateDialog(string version, string url, long size) {
+            Version = version;
+            Url = url;
+            InitializeComponent();
+            VersionText.Text = version;
+            SizeText.Text = $"{size / 1024 / 1024}";
+        }
+
+        private async void InstallButton_Click(object sender, RoutedEventArgs evt) {
+            Description.Visibility = Visibility.Hidden;
+            ProgressBar.Visibility = Visibility.Visible;
+            InstallButton.IsEnabled = false;
+            await Install();
+            DialogResult = true;
+            Close();
+        }
+
+        public async Task Install() {
+            var fileName = Path.Combine(App.TempPath, $"Elwig-{Version}.exe");
+            {
+                using var stream = new FileStream(fileName, FileMode.Create);
+                using var client = new HttpClient() {
+                    Timeout = TimeSpan.FromSeconds(5),
+                };
+                await client.DownloadAsync(Url, stream, new Progress<double>(p => {
+                    ProgressBar.Value = p * 100.0;
+                }));
+            }
+
+            Process.Start(fileName);
+        }
+    }
+}