Creates a generic System.Windows.Forms.ToolStripDropDown for which events can be defined.
The created System.Windows.Forms.ToolStripDropDown object.
The ToolStripDropDownItem.CreateDefaultDropDown method is automatically called if the ToolStripDropDownItem.DropDown property finds no System.Windows.Forms.ToolStripDropDown associated with the System.Windows.Forms.ToolStripDropDownItem.