public void SetHeaderContent (IWidgetBackend backend)
{
if (toolbar.Child != null) {
WidgetBackend.RemoveChildPlacement (toolbar.Child);
toolbar.Remove (toolbar.Child);
}
if (backend != null) {
toolbar.Child = WidgetBackend.GetWidgetWithPlacement (backend);
toolbar.Show ();
} else {
toolbar.Hide ();
}
}