WPF0019 and WPF0021 should handle RegisterAttached and RegisterAttachedReadOnly

[JohanLarsson]

namespace RoslynSandbox
{
    using System.Windows;
    using System.Windows.Controls;

    public static class Foo
    {
        public static readonly DependencyProperty BarProperty = DependencyProperty.RegisterAttached(
        "Bar",
        typeof(int),
        typeof(Foo),
        new PropertyMetadata(1, OnBarChanged));

        public static void SetBar(this FrameworkElement element, int value) => element.SetValue(BarProperty, value);

        public static int GetBar(this FrameworkElement element) => (int)element.GetValue(BarProperty);

        private static void OnBarChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var control = (DataGrid)d;
        }
    }
}

Will be a bit messy as we need to get the type from the arguments in the getvalue & setvalue methods and can still not be sure.
Perhaps limited value?.