Add files for chapter 1

Chapter01/WinUI.SimpleSample/WinUI3.SimpleSample/MainWindow.xaml

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+<!-- Copyright (c) Microsoft Corporation and Contributors. -->
+<!-- Licensed under the MIT License. -->
+
+<Window
+    x:Class="WinUI3.SimpleSample.MainWindow"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:WinUI3.SimpleSample"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d">
+
+    <Grid x:Name="ParentGrid"
+          HorizontalAlignment="Stretch"
+          VerticalAlignment="Top"
+          Padding="2">
+        <Grid.Resources>
+            <Style TargetType="Button">
+                <Setter Property="BorderThickness"
+                        Value="2"/>
+                <Setter Property="Foreground"
+                        Value="LightGray"/>
+                <Setter Property="BorderBrush"
+                        Value="GhostWhite"/>
+                <Setter Property="Background"
+                        Value="DarkBlue"/>
+            </Style>
+        </Grid.Resources>
+        
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto"/>
+            <RowDefinition Height="Auto"/>
+            <RowDefinition Height="*"/>
+        </Grid.RowDefinitions>
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition Width="Auto"/>
+            <ColumnDefinition Width="*"/>
+        </Grid.ColumnDefinitions>
+        <TextBlock Grid.Row="0" Grid.Column="0"
+                   Text="Name:"
+                   Margin="0,0,2,0"
+                   VerticalAlignment="Center"/>
+        <TextBox Grid.Row="0" Grid.Column="1"
+                 Text="{x:Bind Path=ViewModel.Name, Mode=TwoWay}"/>
+        <TextBlock Grid.Row="1" Grid.Column="0"
+                   Text="Actual Width:"
+                   Margin="0,0,2,0"
+                   VerticalAlignment="Center"/>
+        <TextBlock Grid.Row="1" Grid.Column="1"
+                   Text="{Binding ElementName=ParentGrid, Path=ActualWidth}"/>
+        <StackPanel Grid.Row="2" Grid.Column="1"
+                    Margin="0,4,0,0"
+                    HorizontalAlignment="Right"
+                    VerticalAlignment="Top"
+                    Orientation="Horizontal">
+            <PersonPicture Initials="MS" Margin="0,0,8,0"/>
+            <DropDownButton Content="Submit" Margin="0,0,4,0">
+                <DropDownButton.Flyout>
+                    <MenuFlyout Placement="Bottom">
+                        <MenuFlyoutItem Text="Submit + Print"/>
+                        <MenuFlyoutItem Text="Submit + Email"/>
+                    </MenuFlyout>
+                </DropDownButton.Flyout>
+            </DropDownButton>
+            <Button Content="Cancel"/>
+        </StackPanel>
+    </Grid>
+</Window>