Replace MainWindow with plain Window in sample projects (#2063)

<!--- Provide a general summary of your changes in the Title above -->

## Description
<!--- Describe your changes in detail -->
This PR updates `WindowHelper.CreateWindow()` to return a plain `Window`
instead of `MainWindow`, which was tightly coupled to several static
members.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here. -->

Fixes #2062.

## How Has This Been Tested?
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to -->
<!--- see how your change affects other areas of the code, etc. -->
Manually tested.

## Screenshots (if appropriate):

<img width="1115" height="793" alt="image"
src="https://github.com/user-attachments/assets/ecd95d4d-a550-4d58-9a85-ec5243f304f2"
/>

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

Author: AndrewKeepCoding

WinUIGallery/Helpers/WindowHelper.cs

@@ -19,7 +19,7 @@ public partial class WindowHelper
 {
     static public Window CreateWindow()
     {
-        MainWindow newWindow = new MainWindow();
+        var newWindow = new Window();
         TrackWindow(newWindow);
         return newWindow;
     }

WinUIGallery/Samples/ControlPages/CreateMultipleWindowsPage.xaml

@@ -20,7 +20,7 @@
 
     <StackPanel>
         <controls:ControlExample CSharpSource="Window/CreateWindowSample1.txt" HeaderText="Create single threaded Multiple Top level Windows(MTW).">
-            <Button x:Name="Control1" Click="createNewWindow_Click">Create new Window</Button>
+            <Button x:Name="Control1" Click="CreateNewWindow_Click">Create new Window</Button>
         </controls:ControlExample>
     </StackPanel>
 </Page>

WinUIGallery/Samples/ControlPages/CreateMultipleWindowsPage.xaml.cs

@@ -3,8 +3,9 @@
 
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media;
+using Windows.Graphics;
 using WinUIGallery.Helpers;
-using WinUIGallery.Pages;
 
 namespace WinUIGallery.ControlPages;
 
@@ -15,14 +16,30 @@ public CreateMultipleWindowsPage()
         this.InitializeComponent();
     }
 
-    private void createNewWindow_Click(object sender, RoutedEventArgs e)
+    private void CreateNewWindow_Click(object sender, RoutedEventArgs e)
     {
-        var newWindow = new MainWindow();
-        WindowHelper.TrackWindow(newWindow);
-        newWindow.Activate();
+        var childWindow = new Window()
+        {
+            ExtendsContentIntoTitleBar = true,
+            SystemBackdrop = new MicaBackdrop(),
+            Content = new Page()
+            {
+                Content = new TextBlock()
+                {
+                    Text = "New child window!",
+                    HorizontalAlignment = HorizontalAlignment.Center,
+                    VerticalAlignment = VerticalAlignment.Center,
+                },
+                // Get the theme from the parent.
+                RequestedTheme = this.ActualTheme,
+            }
+        };
 
-        var targetPageType = typeof(HomePage);
-        string targetPageArguments = string.Empty;
-        newWindow.Navigate(targetPageType, targetPageArguments);
+        // We need to track the new window so it can be closed when the app is closing,
+        // otherwise it will crash the app.
+        // This is also used to change the theme for all windows when the app theme changes.
+        WindowHelper.TrackWindow(childWindow);
+        childWindow.AppWindow.ResizeClient(new SizeInt32(500, 500));
+        childWindow.Activate();
     }
 }

WinUIGallery/Samples/SampleCode/Window/CreateWindowSample1.txt

@@ -1,11 +1,20 @@
-// C# code to create a new window
-var newWindow = WindowHelper.CreateWindow();
-var rootPage = new NavigationRootPage();
-rootPage.RequestedTheme = ThemeHelper.RootTheme;
-newWindow.Content = rootPage;
-newWindow.Activate();
+// Ensure you close the child window before closing the parent window to avoid application crash.
+var childWindow = new Window()
+{
+    ExtendsContentIntoTitleBar = true,
+    SystemBackdrop = new MicaBackdrop(),
+    Content = new Page()
+    {
+        Content = new TextBlock()
+        {
+            Text = "New child window!",
+            HorizontalAlignment = HorizontalAlignment.Center,
+            VerticalAlignment = VerticalAlignment.Center,
+        },
+        // Get the theme from the parent.
+        RequestedTheme = this.ActualTheme,
+    }
+};
 
-// C# code to navigate in the new window
-var targetPageType = typeof(HomePage);
-string targetPageArguments = string.Empty;
-rootPage.Navigate(targetPageType, targetPageArguments);
+childWindow.AppWindow.ResizeClient(new SizeInt32(500, 500));
+childWindow.Activate();