ToolStripSplitButton Checked 效果

ToolStripSplitButton 系统默认没有Checked效果，MS又说他们没时间改。

只能自己处理一下了。

重载OnPaint(PaintEventArgs e)

if（Checked）

                    Rectangle rect = ButtonBounds;
                    using (Brush br = new LinearGradientBrush(rect,
                                                              ProfessionalColors.ButtonCheckedGradientBegin,
                                                              ProfessionalColors.ButtonCheckedGradientEnd,
                                                              LinearGradientMode.Vertical))
                    {
                        e.Graphics.FillRectangle(br, rect);
                    }

                    rect.Inflate(-1, -1);
                    e.Graphics.DrawRectangle(new System.Drawing.Pen(ProfessionalColors.ButtonSelectedHighlightBorder), rect);
                    base.OnPaint(e);