Draws an expander as used in. @x and @y specify the center the expander. The size of the expander is determined by the "expander-size" style property of @widget. (If widget is not specified or doesn't have an "expander-size" property, an unspecified default size will be used, since the caller doesn't have sufficient information to position the expander, this is likely not useful.) The expander is expander_size pixels tall in the collapsed position and expander_size pixels wide in the expanded position.