[#8] Add auto update checker

Elwig/Dialogs/UpdateDialog.xaml

@@ -0,0 +1,33 @@
+<Window x:Class="Elwig.Dialogs.UpdateDialog"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        mc:Ignorable="d"
+        ResizeMode="NoResize"
+        ShowInTaskbar="False"
+        Topmost="True"
+        WindowStartupLocation="CenterOwner"
+        Title="Neues Update verfügbar - Elwig" Height="180" Width="400">
+    <Grid>
+        <TextBlock x:Name="Description" FontSize="14" Margin="0,0,0,30"
+                   HorizontalAlignment="Center" VerticalAlignment="Center" TextAlignment="Center">
+            Version <Run x:Name="VersionText" FontWeight="Bold">0.0.0</Run> von Elwig ist verfügbar!<LineBreak/>
+            Soll das Update heruntergeladen und<LineBreak/>
+            installiert werden? (ca. <Run x:Name="SizeText">100</Run> MB)<LineBreak/>
+            <Run FontWeight="Bold">Achtung</Run>: Elwig wird dabei geschlossen!
+        </TextBlock>
+
+        <ProgressBar x:Name="ProgressBar" Margin="0,0,0,27" Visibility="Hidden"
+                     HorizontalAlignment="Center" VerticalAlignment="Center"
+                     Height="27" Width="300" SnapsToDevicePixels="True"/>
+
+        <Button x:Name="CancelButton" Content="Abbrechen" Margin="10,10,115,10" IsCancel="True" IsDefault="True"
+                FontSize="14" HorizontalAlignment="Right" VerticalAlignment="Bottom"
+                Width="100" Height="27"/>
+        <Button x:Name="InstallButton" Content="Installieren" Margin="10,10,10,10"
+                FontSize="14" HorizontalAlignment="Right" VerticalAlignment="Bottom"
+                Width="100" Height="27"
+                Click="InstallButton_Click"/>
+    </Grid>
+</Window>

Elwig/Dialogs/UpdateDialog.xaml.cs

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+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);
+        }
+    }
+}