CmdPal: Filters for DynamicListPage? Yes, please. (#40783)

Closes: #40382

## To-do list

- [x] Add support for "single-select" filters to DynamicListPage
- [x] Filters can contain icons
- [x] Filter list can contain separators
- [x] Update Windows Services built-in extension to support filtering by
all, started, stopped, and pending services
- [x] Update SampleExtension dynamic list sample to filter.

## Example of filters in use

```C#
internal sealed partial class ServicesListPage : DynamicListPage
{
    public ServicesListPage()
    {
        Icon = Icons.ServicesIcon;
        Name = "Windows Services";

        var filters = new ServiceFilters();
        filters.PropChanged += Filters_PropChanged;
        Filters = filters;
    }

    private void Filters_PropChanged(object sender, IPropChangedEventArgs args) => RaiseItemsChanged();

    public override void UpdateSearchText(string oldSearch, string newSearch) => RaiseItemsChanged();

    public override IListItem[] GetItems()
    {
       // ServiceHelper.Search knows how to filter based on the CurrentFilterIds provided
        var items = ServiceHelper.Search(SearchText, Filters.CurrentFilterIds).ToArray();

        return items;
    }
}

public partial class ServiceFilters : Filters
{
    public ServiceFilters()
    {
        // This would be a default selection. Not providing this will cause the filter
        // control to display the "Filter" placeholder text.
        CurrentFilterIds = ["all"];
    }

    public override IFilterItem[] GetFilters()
    {
        return [
            new Filter() { Id = "all", Name = "All Services" },
            new Separator(),
            new Filter() { Id = "running", Name = "Running", Icon = Icons.GreenCircleIcon },
            new Filter() { Id = "stopped", Name = "Stopped", Icon = Icons.RedCircleIcon },
            new Filter() { Id = "paused", Name = "Paused", Icon = Icons.PauseIcon },
        ];
    }
}
```

## Current example of behavior


https://github.com/user-attachments/assets/2e325763-ad3a-4445-bbe2-a840df08d0b3

---------

Co-authored-by: Mike Griese <migrie@microsoft.com>

src/modules/cmdpal/Microsoft.CmdPal.UI/Converters/ContextItemTemplateSelector.cs

@@ -28,7 +28,7 @@ internal sealed partial class ContextItemTemplateSelector : DataTemplateSelector
         {
             li.IsEnabled = true;
 
-            if (item is SeparatorContextItemViewModel)
+            if (item is SeparatorViewModel)
             {
                 li.IsEnabled = false;
                 li.AllowFocusWhenDisabled = false;

src/modules/cmdpal/Microsoft.CmdPal.UI/Converters/FilterTemplateSelector.cs

@@ -0,0 +1,36 @@
+// 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.
+
+using Microsoft.CmdPal.Core.ViewModels;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+
+namespace Microsoft.CmdPal.UI;
+
+internal sealed partial class FilterTemplateSelector : DataTemplateSelector
+{
+    public DataTemplate? Default { get; set; }
+
+    public DataTemplate? Separator { get; set; }
+
+    protected override DataTemplate? SelectTemplateCore(object item, DependencyObject dependencyObject)
+    {
+        DataTemplate? dataTemplate = Default;
+
+        if (dependencyObject is ComboBoxItem comboBoxItem)
+        {
+            comboBoxItem.IsEnabled = true;
+
+            if (item is SeparatorViewModel)
+            {
+                comboBoxItem.IsEnabled = false;
+                comboBoxItem.AllowFocusWhenDisabled = false;
+                comboBoxItem.AllowFocusOnInteraction = false;
+                dataTemplate = Separator;
+            }
+        }
+
+        return dataTemplate;
+    }
+}