[controls] fix memory leak in Application.OpenWindow()

Context: dotnet#10029

I could reproduce a memory leak by doing:

    App.Current.OpenWindow(new Window { Page = new ContentPage() });

I found that `App.Current._requestedWindows` just held onto every
`Window` object forever.

This is a perfect example of where using `ConditionalWeakTable` solves
the issue:

https://learn.microsoft.com/dotnet/api/system.runtime.compilerservices.conditionalweaktable-2

The only other change I made was to use `Guid.ToString("n")` as it
removes `{`, `}`, and `-` characters from the string.

Note this doesn't fully solve dotnet#10029, as I still see something else
holding onto `Window` in my example.

This is WIP, as I'm interested in what my `WindowsDoNotLeak` test will
do on all platforms.

src/Controls/src/Core/HandlerImpl/Application/Application.Impl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using Microsoft.Extensions.Logging;
 using Microsoft.Maui.ApplicationModel;
 using Microsoft.Maui.Devices;
@@ -12,7 +13,7 @@ public partial class Application : IApplication
 		const string MauiWindowIdKey = "__MAUI_WINDOW_ID__";
 
 		readonly List<Window> _windows = new();
-		readonly Dictionary<string, Window> _requestedWindows = new();
+		readonly ConditionalWeakTable<string, Window> _requestedWindows = new();
 		ILogger<Application>? _logger;
 
 		ILogger<Application>? Logger =>
@@ -30,7 +31,10 @@ IWindow IApplication.CreateWindow(IActivationState? activationState)
 			if (activationState?.State?.TryGetValue(MauiWindowIdKey, out var requestedWindowId) ?? false)
 			{
 				if (requestedWindowId != null && _requestedWindows.TryGetValue(requestedWindowId, out var w))
+				{
 					window = w;
+					_requestedWindows.Remove(requestedWindowId);
+				}
 			}
 
 			// create a new one if there is no pending windows
@@ -86,9 +90,8 @@ internal void RemoveWindow(Window window)
 
 		public virtual void OpenWindow(Window window)
 		{
-			var id = Guid.NewGuid().ToString();
-
-			_requestedWindows[id] = window;
+			var id = Guid.NewGuid().ToString("n");
+			_requestedWindows.Add(id, window);
 
 			var state = new PersistedState
 			{

src/Controls/tests/DeviceTests/Elements/Window/WindowTests.cs

@@ -198,5 +198,23 @@ await CreateHandlerAndAddToWindow<WindowHandlerStub>(window, async (handler) =>
 		}
 #endif
 
+		[Fact]
+		public async Task WindowsDoNotLeak()
+		{
+			var window = new Window(new ContentPage());
+			var windowReference = new WeakReference(window);
+
+			await CreateHandlerAndAddToWindow<WindowHandlerStub>(window, _ =>
+			{
+				new Application().OpenWindow(window);
+			});
+
+			window = null;
+			await Task.Yield();
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+
+			Assert.False(windowReference.IsAlive, "Window wasn't collected");
+		}
 	}
 }