chore: Custom window sample

Author: MartinZikmund

UI/WindowingSamples/WindowingSamples/CustomWindow.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<Window
+    x:Class="WindowingSamples.CustomWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:controls="using:WindowingSamples.Controls"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d">
+
+  <Grid x:Name="RootGrid">
+    <Grid.RowDefinitions>
+      <RowDefinition Height="Auto" />
+      <RowDefinition Height="*" />
+    </Grid.RowDefinitions>
+    <controls:WindowTitleControl Text="Custom window" Glyph="&#xE161;" />
+
+    
+    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" Grid.Row="1" Spacing="8">
+      <TextBlock x:Name="WindowText" HorizontalAlignment="Center" Text="This is custom window" FontSize="40" />
+      <Button HorizontalAlignment="Center" Click="CloseClick">Close</Button>
+    </StackPanel>
+  </Grid>
+</Window>

UI/WindowingSamples/WindowingSamples/CustomWindow.xaml.cs

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+using Microsoft.UI.Windowing;
+using Windows.UI;
+
+namespace WindowingSamples;
+
+/// <summary>
+/// An empty window that can be used on its own or navigated to within a Frame.
+/// </summary>
+public sealed partial class CustomWindow : Window
+{
+    private static int _customWindowCounter = 0;
+
+    public CustomWindow()
+    {
+        InitializeComponent();
+
+        var random = Random.Shared;
+        RootGrid.Background = new SolidColorBrush(
+            Color.FromArgb(
+                255, 
+                (byte)random.Next(0,150), 
+                (byte)random.Next(0, 150), 
+                (byte)random.Next(0,150)));
+
+        var text = $"Custom Window {++_customWindowCounter}";
+        Title = text;
+        WindowText.Text = text;
+    }
+
+    public void CloseClick(object sender, RoutedEventArgs args) => Close();
+}