[API Proposal]: Make ItemsControl::OnBringItemIntoView protected

### Background and motivation

When inheriting from ItemsControl, there is no direct way to use the existing API, but it can be useful to have the possibility to use it.

### API Proposal

```csharp
protected internal object OnBringItemIntoView(object arg)
{
//
}
```

### API Usage

I just want to be able use the same code as in ListBox
```csharp
public void ScrollIntoView(object item)
{
if (ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
{
OnBringItemIntoView(item);
}
else
{
// The items aren't generated, try at a later time
Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new DispatcherOperationCallback(OnBringItemIntoView), item);
}
}
```

### Alternative Designs

Perhaps a better alternative would be to bring the public method `ListBox::ScrollIntoView` up to `ItemsControl`. But make it virtual, as e.g. `ComboBox` needs some special love (see `ComboBoxAutomationProvider::ScrollItemIntoView`).

### Risks

Could break Applications when developers have a derived `ItemsControl` where they already have a `OnBringItemIntoView` method with the same signature. Or in case of the alternative a `ScrollIntoView` method.