Inheritance: WindowlessControls.WindowlessPaintControl
 protected override void OnSetupDockPanel(DockPanel dockPanel)
 {
     WindowlessDropDownArrow arrow = new WindowlessDropDownArrow();
     arrow.Layout = new DockLayout(new LayoutMeasurement(0, LayoutUnit.Star), DockStyle.Right);
     dockPanel.Controls.Add(arrow);
     base.OnSetupDockPanel(dockPanel);
 }