SelectedItems property on DataGrid

The DataGrid has a property SelectedItems property which can not be used in WPF like 'SelectedItem' because it has no dependency property. I would love to see this be added to WPF.

See as well: https://stackoverflow.com/questions/9880589/bind-to-selecteditems-from-datagrid-or-listbox-in-mvvm
Here the DataGrid is used with SelectionMode "Extended" and using SelectionUnit="FullRow" --> with this configuration I think there should be no issue adding this functionality.

Here a raw template for a simple extension (only supporting to read the 'SelectedItems' property):

``` c#
public class CustomDataGrid : DataGrid
{
public static readonly DependencyProperty SelectedItemsProperty = DependencyProperty.Register(nameof(SelectedItems), typeof(IList), typeof(CustomDataGrid), new PropertyMetadata(default(IList), OnSelectedItemsPropertyChanged));

protected override void OnSelectionChanged(SelectionChangedEventArgs e)
{
base.OnSelectionChanged(e);
SetValue(SelectedItemsProperty, base.SelectedItems);
}

public new IList SelectedItems
{
get => (IList)GetValue(SelectedItemsProperty);
set => throw new Exception("This property is read-only. To bind to it you must use 'Mode=OneWayToSource'.");
}

private static void OnSelectedItemsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
((CustomDataGrid)d).OnSelectedItemsChanged((IList)e.OldValue, (IList)e.NewValue);
}

protected virtual void OnSelectedItemsChanged(IList oldSelectedItems, IList newSelectedItems)
{
}
}
```