[ios/catalyst] fix memory leak in gestures

[jonathanpeppers]

Related: #21089
Context: jonathanpeppers/iOSMauiCollectionView@547b7fa

Doing something like this and running on iOS or Catalyst:

<CollectionView.ItemTemplate>
    <DataTemplate>
        <Label Text="{Binding Name}">
            <Label.GestureRecognizers>
                <TapGestureRecognizer Command="{Binding BindingContext.Command}" />
            </Label.GestureRecognizers>
        </Label>
    </DataTemplate>
</CollectionView.ItemTemplate>

Causes a memory leak.

I could reproduce this in a test that is a bit more elaborate than #21089, showing issues for different types of gestures:

• Usage of ShouldReceiveTouch creates a cycle:

  • GesturePlatformManager -> UIGestureRecognizer.ShouldReceiveTouch -> GesturePlatformManager

  • We can move ShouldReceiveTouch to a "proxy" class, and avoid the cycle.

• PanGestureRecognizer has closures that cause the same cycle:

  • this.PlatformView -> eventTracker?.PlatformView

  • panRecognizer -> ((PanGestureRecognizer)panGestureRecognizer)

  • These already had a WeakReference to use instead, but we could just make use of them.

There may be a general problem with CollectionView in how it doesn't tear down GesturePlatformManager in the same way for children. But in either case, the problems above are cycles that should be broken.

[jonathanpeppers]

C# 11 and higher does this for you now: dotnet/roslyn#5835

[jonathanpeppers]

This indented a big chunk of code, so this makes it easier to review: