The Xamarin.Forms.DataTemplate for group headers, or null.
Use this property to define a template for a Xamarin.Forms.Cell that will be used as the header for groups in this Xamarin.Forms.ListView. The Xamarin.BindableObject.BindingContext will be the IEnumerable for each group.
GroupHeaderTemplate is mutually exclusive with ListView.GroupDisplayBinding. Setting this property will set ListView.GroupDisplayBinding to null.
Empty groups will still display a group header.