The index of the currently selected item.
If set to [const@Gtk.INVALID_LIST_POSITION], no item is selected.
If [property@Sidebar:mode] is set to [enum@Adw.SidebarMode.page], the selection is invisible, but still tracked, indicating which item will be selected once the mode is changed to [enum@Adw.SidebarMode.sidebar].
See also: [property@Sidebar:selected-item].