[controls] fix memory leak with Grid Row/ColumnDefinitions

Context: dotnet#15860

In a customer's app, they have the setup:

    <!-- Resources/Styles/Styles.xaml -->
    <Style TargetType="Grid" x:Key="GridStyleWithColumnDefinitions">
        <Setter Property="ColumnDefinitions" Value="18,*"/>
    </Style>

Then a page with:

    <Grid Style="{StaticResource GridStyleWithColumnDefinitions}" />

Navigating forward & back from this page would show that the entire
`Page` would live forever! What is interesting, is simply removing the
Grid's `Style` solved the problem?!?

I narrowed this down to a unit test:

    [Fact]
    public async Task ColumnDefinitionDoesNotLeak()
    {
        // Long-lived column, like from a Style in App.Resources
        var column = new ColumnDefinition();
        WeakReference reference;

        {
            var grid = new Grid();
            grid.ColumnDefinitions.Add(column);
            reference = new(grid);
        }

        await Task.Yield();
        GC.Collect();
        GC.WaitForPendingFinalizers();

        Assert.False(reference.IsAlive, "Grid should not be alive!");
    }

https://github.com/dotnet/maui/blob/cda3eb3381cfb686567ed05e3eb8e8f26c02d785/src/Controls/src/Core/Layout/Grid.cs#L20

`Grid` subscribes to `DefinitionCollection<T>.ItemSizeChanged`, and in
this case the `DefinitionCollection<T>` lived indefinitely in an
application-wide `ResourceDictionary`.

The simplest solution is to make the `ItemSizeChanged` event use
`WeakEventManager` as was done for other events like
`Application.RequestedThemeChanged`:

https://github.com/dotnet/maui/blob/cda3eb3381cfb686567ed05e3eb8e8f26c02d785/src/Controls/src/Core/Application/Application.cs#L221-L225

Since MAUI owns the property, we can use the simple solution here.

This is one issue solved in the customer app, but I will need to retest
the app in dotnet#15860 to see if there are further issues.

src/Controls/src/Core/DefinitionCollection.cs

@@ -7,6 +7,7 @@ namespace Microsoft.Maui.Controls
 {
 	public class DefinitionCollection<T> : IList<T>, ICollection<T> where T : IDefinition
 	{
+		readonly WeakEventManager _weakEventManager = new WeakEventManager();
 		readonly List<T> _internalList;
 
 		internal DefinitionCollection() => _internalList = new List<T>();
@@ -101,13 +102,15 @@ public void RemoveAt(int index)
 			OnItemSizeChanged(this, EventArgs.Empty);
 		}
 
-		public event EventHandler ItemSizeChanged;
+		public event EventHandler ItemSizeChanged
+		{
+			add => _weakEventManager.AddEventHandler(value);
+			remove => _weakEventManager.RemoveEventHandler(value);
+		}
 
 		void OnItemSizeChanged(object sender, EventArgs e)
 		{
-			EventHandler eh = ItemSizeChanged;
-			if (eh != null)
-				eh(this, EventArgs.Empty);
+			_weakEventManager.HandleEvent(this, e, nameof(ItemSizeChanged));
 		}
 	}
 }

src/Controls/tests/Core.UnitTests/Layouts/GridLayoutTests.cs

@@ -1,4 +1,6 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
 using NSubstitute;
 using Xunit;
 
@@ -184,5 +186,45 @@ public void ColumnDefinitionsGetBindingContext()
 
 			Assert.Equal(def2.BindingContext, context);
 		}
+
+		[Fact]
+		public async Task ColumnDefinitionDoesNotLeak()
+		{
+			// Long-lived column, like from a Style in App.Resources
+			var column = new ColumnDefinition();
+			WeakReference reference;
+
+			{
+				var grid = new Grid();
+				grid.ColumnDefinitions.Add(column);
+				reference = new(grid);
+			}
+
+			await Task.Yield();
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+
+			Assert.False(reference.IsAlive, "Grid should not be alive!");
+		}
+
+		[Fact]
+		public async Task RowDefinitionDoesNotLeak()
+		{
+			// Long-lived row, like from a Style in App.Resources
+			var row = new RowDefinition();
+			WeakReference reference;
+
+			{
+				var grid = new Grid();
+				grid.RowDefinitions.Add(row);
+				reference = new(grid);
+			}
+
+			await Task.Yield();
+			GC.Collect();
+			GC.WaitForPendingFinalizers();
+
+			Assert.False(reference.IsAlive, "Grid should not be alive!");
+		}
 	}
 }