CmdPal: Port the devhome perf widgets to cmdpal (#45217)

Pretty direct port of the code, to prove it works.

There's definitely some improvement to be made here, esp WRT to the
network and GPU listing - networks should all just be listed. Or at
least automatically track the active one. And GPU should aggregate a
bunch of stats.

And we can probably add the details to these list items.

But most importantly, _it works_.

re: #45201

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.PerformanceMonitor/OnLoadStaticPage.cs

@@ -0,0 +1,123 @@
+// 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 System;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.CommandPalette.Extensions.Toolkit;
+using Windows.Foundation;
+
+namespace Microsoft.CmdPal.Ext.PerformanceMonitor;
+
+#pragma warning disable SA1402 // File may only contain a single type
+
+/// <summary>
+/// Helper class for creating ListPage's which can listen for when they're
+/// loaded and unloaded. This works because CmdPal will attach an event handler
+/// to the ItemsChanged event when the page is added to the UI, and remove it
+/// when the page is removed from the UI.
+///
+/// Subclasses should override the Loaded and Unloaded methods to start/stop
+/// any background work needed to populate the page.
+/// </summary>
+internal abstract partial class OnLoadStaticListPage : OnLoadBasePage, IListPage
+{
+    private string _searchText = string.Empty;
+
+    public virtual string PlaceholderText { get; set => SetProperty(ref field, value); } = string.Empty;
+
+    public virtual string SearchText { get => _searchText; set => SetProperty(ref _searchText, value); }
+
+    public virtual bool ShowDetails { get; set => SetProperty(ref field, value); }
+
+    public virtual bool HasMoreItems { get; set => SetProperty(ref field, value); }
+
+    public virtual IFilters? Filters { get; set => SetProperty(ref field, value); }
+
+    public virtual IGridProperties? GridProperties { get; set => SetProperty(ref field, value); }
+
+    public virtual ICommandItem? EmptyContent { get; set => SetProperty(ref field, value); }
+
+    public void LoadMore()
+    {
+    }
+
+    protected void SetSearchNoUpdate(string newSearchText)
+    {
+        _searchText = newSearchText;
+    }
+
+    public abstract IListItem[] GetItems();
+}
+
+/// <summary>
+/// Helper class for creating ContentPage's which can listen for when they're
+/// loaded and unloaded. This works because CmdPal will attach an event handler
+/// to the ItemsChanged event when the page is added to the UI, and remove it
+/// when the page is removed from the UI.
+///
+/// Subclasses should override the Loaded and Unloaded methods to start/stop
+/// any background work needed to populate the page.
+/// </summary>
+internal abstract partial class OnLoadContentPage : OnLoadBasePage, IContentPage
+{
+    public virtual IDetails? Details { get; set => SetProperty(ref field, value); }
+
+    public virtual IContextItem[] Commands { get; set => SetProperty(ref field, value); } = [];
+
+    public abstract IContent[] GetContent();
+}
+
+internal abstract partial class OnLoadBasePage : Page
+{
+    private int _loadCount;
+
+#pragma warning disable CS0067 // The event is never used
+
+    private event TypedEventHandler<object, IItemsChangedEventArgs>? InternalItemsChanged;
+#pragma warning restore CS0067 // The event is never used
+
+    public event TypedEventHandler<object, IItemsChangedEventArgs> ItemsChanged
+    {
+        add
+        {
+            InternalItemsChanged += value;
+            if (_loadCount == 0)
+            {
+                Loaded();
+            }
+
+            _loadCount++;
+        }
+
+        remove
+        {
+            InternalItemsChanged -= value;
+            _loadCount--;
+            _loadCount = Math.Max(0, _loadCount);
+            if (_loadCount == 0)
+            {
+                Unloaded();
+            }
+        }
+    }
+
+    protected abstract void Loaded();
+
+    protected abstract void Unloaded();
+
+    protected void RaiseItemsChanged(int totalItems = -1)
+    {
+        try
+        {
+            // TODO #181 - This is the same thing that BaseObservable has to deal with.
+            InternalItemsChanged?.Invoke(this, new ItemsChangedEventArgs(totalItems));
+        }
+        catch
+        {
+        }
+    }
+}
+
+
+#pragma warning restore SA1402 // File may only contain a single type