private void 縱向平鋪ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.TileVertical); 
        }

        private void 橫向平鋪ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.TileHorizontal); 
        }

        private void 層疊窗口ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.LayoutMdi(MdiLayout.Cascade);  
        }