[controls] fix memory leak in Window

Fixes: 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.

After these changes, I still found `Window` objects hanging around
that appeared to be held onto by many other objects.

Then I reviewed:

    void IWindow.Destroying()
    {
        SendWindowDisppearing();
        Destroying?.Invoke(this, EventArgs.Empty);
        OnDestroying();

        Application?.RemoveWindow(this);
        // This wasn't here!
        Handler?.DisconnectHandler();
    }

It appears that upon `Window`'s closing, we didn't have any code that
"disconnected" the MAUI handler. After this change, I could see
`Window` objects properly go away.

I added a unit test as well.

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/src/Core/HandlerImpl/Window/Window.Impl.cs

@@ -498,6 +498,7 @@ void IWindow.Destroying()
 			OnDestroying();
 
 			Application?.RemoveWindow(this);
+			Handler?.DisconnectHandler();
 		}
 
 		void IWindow.Resumed()

src/Controls/tests/Core.UnitTests/WindowsTests.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.Maui.Graphics;
@@ -621,5 +622,27 @@ public void MinDimensionsArePassedToCoreCorrectly(double inW, double inH, double
 			Assert.Equal(outW, coreWindow.MinimumWidth);
 			Assert.Equal(outH, coreWindow.MinimumHeight);
 		}
+
+		[Fact]
+		public async Task WindowDoesNotLeak()
+		{
+			var application = new Application();
+			WeakReference reference;
+
+			// Scope for window
+			{
+				var window = new Window { Page = new ContentPage() };
+				reference = new WeakReference(window);
+				application.OpenWindow(window);
+				((IWindow)window).Destroying();
+			}
+
+			// GC
+			await Task.Yield();
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+
+			Assert.False(reference.IsAlive, "Window should not be alive!");
+		}
 	}
 }