Default is null for Plain tables and non-null for Grouped style tables.
This value can be null.
UIKit.UITableView adds the value of this proerty as a subview only when the cell is selected. It is added as a subview directly above the UITableView.BackgroundView if that is not null, otherwise behind all other views.
If UITableViewCell.SetSelected(bool, bool) is called, this view is animated in and out with an alpha fade.