Fixes: dotnet#12344
Fixes: dotnet#13557
Context: https://github.com/dotnet-presentations/dotnet-maui-workshop

While testing the `Monkey Finder` sample, I found the following
scenario causes an issue:

1. Declare a `{StaticResource}` `Brush` at the `Application` level,
   with a lifetime of the entire application.

2. Set `Background` on an item in a `CollectionView`, `ListView`, etc.

3. Scroll a lot, navigate away, etc.

4. The `Brush` will hold onto any `View`'s indefinitely.

The core problem here being `VisualElement` does:

    void NotifyBackgroundChanges()
    {
        if (Background is ImmutableBrush)
            return;

        if (Background != null)
        {
            Background.Parent = this;
            Background.PropertyChanged += OnBackgroundChanged;

            if (Background is GradientBrush gradientBrush)
                gradientBrush.InvalidateGradientBrushRequested += InvalidateGradientBrushRequested;
        }
    }

If no user code sets `Background` to `null`, these events remain
subscribed.

To fix this:

1. Create a `WeakNotifyCollectionChangedProxy` type for event subscription.

2. Don't set `Background.Parent = this`

~~ General Cleanup ~~

Through doing other fixes related to memory leaks & C# events, we've
started to gain a collection of `WeakEventProxy`-related types.

I created some core `internal` types to be reused:

* `abstract WeakEventProxy<TSource, TEventHandler>`
* `WeakNotifyCollectionChangedProxy`

The following classes now make use of these new shared types:

* `BindingExpression`
* `BindableLayout`
* `ListProxy`
* `VisualElement`

This should hopefully reduce mistakes and reuse code in this area.

~~ Concerns ~~

Since, we are no longer doing:

    Background.Parent = this;

This is certainly a behavior change. It is now replaced with:

    SetInheritedBindingContext(Background, BindingContext);

I had to update one unit test that was asserting `Background.Parent`,
which can assert `Background.BindingContext` instead.

As such, this change is definitely sketchy and I wouldn't backport to
.NET 7 immediately. We might test it out in a .NET 8 preview first.