update fltk uimenu position when adding to figure * libinterp/dldfcn/__init_fltk__.cc (fltk_uimenu::update_position): new function (fltk_uimenu::add_to_menu): delay creating menus with pos <= 0, assign position value based on number of menus created. * scripts/gui/uimenu.m: assert position of 0 instead of 9