[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/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