[#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>