[CmdPal] Single settings window (#38069)

## Summary of the Pull Request

Make settings window single.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] **Closes:** https://github.com/zadjii-msft/PowerToys/issues/581

## Validation Steps Performed

Manual test.
- Open settings windows multiple times
- Activate minimized settings window

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Messages/SettingsWindowClosedMessage.cs

@@ -0,0 +1,9 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CmdPal.UI.ViewModels.Messages;
+
+public record SettingsWindowClosedMessage
+{
+}

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -35,6 +35,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     IRecipient<ClearSearchMessage>,
     IRecipient<HandleCommandResultMessage>,
     IRecipient<LaunchUriMessage>,
+    IRecipient<SettingsWindowClosedMessage>,
     INotifyPropertyChanged
 {
     private readonly DispatcherQueue _queue = DispatcherQueue.GetForCurrentThread();
@@ -50,6 +51,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
 
     private readonly Lock _invokeLock = new();
     private Task? _handleInvokeTask;
+    private SettingsWindow? _settingsWindow;
 
     public ShellViewModel ViewModel { get; private set; } = App.Current.Services.GetService<ShellViewModel>()!;
 
@@ -65,6 +67,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         WeakReferenceMessenger.Default.Register<HandleCommandResultMessage>(this);
         WeakReferenceMessenger.Default.Register<OpenSettingsMessage>(this);
         WeakReferenceMessenger.Default.Register<HotkeySummonMessage>(this);
+        WeakReferenceMessenger.Default.Register<SettingsWindowClosedMessage>(this);
 
         WeakReferenceMessenger.Default.Register<ShowDetailsMessage>(this);
         WeakReferenceMessenger.Default.Register<HideDetailsMessage>(this);
@@ -396,8 +399,12 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
             // Also hide our details pane about here, if we had one
             HideDetails();
 
-            var settingsWindow = new SettingsWindow();
+            if (_settingsWindow == null)
-            settingsWindow.Activate();
+            {
+                _settingsWindow = new SettingsWindow();
+            }
+
+            _settingsWindow.Activate();
 
             WeakReferenceMessenger.Default.Send<UpdateCommandBarMessage>(new(null));
         });
@@ -458,6 +465,8 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         _ = DispatcherQueue.TryEnqueue(() => SummonOnUiThread(message));
     }
 
+    public void Receive(SettingsWindowClosedMessage message) => _settingsWindow = null;
+
     private void SummonOnUiThread(HotkeySummonMessage message)
     {
         var settings = App.Current.Services.GetService<SettingsModel>()!;

src/modules/cmdpal/Microsoft.CmdPal.UI/Settings/SettingsWindow.xaml

@@ -9,6 +9,7 @@
     xmlns:ui="using:CommunityToolkit.WinUI"
     Title="SettingsWindow"
     Activated="Window_Activated"
+    Closed="Window_Closed"
     mc:Ignorable="d">
     <Window.SystemBackdrop>
         <MicaBackdrop />

src/modules/cmdpal/Microsoft.CmdPal.UI/Settings/SettingsWindow.xaml.cs

@@ -105,6 +105,11 @@ public sealed partial class SettingsWindow : Window,
         WeakReferenceMessenger.Default.Send<WindowActivatedEventArgs>(args);
     }
 
+    private void Window_Closed(object sender, WindowEventArgs args)
+    {
+        WeakReferenceMessenger.Default.Send<SettingsWindowClosedMessage>();
+    }
+
     public void Receive(QuitMessage message)
     {
         // This might come in on a background thread