small update of ncurses-st-menu

ncurses-st-menu is small library over ncurses for pulldown and menubar menu. Stripped size of library has 43kB, so it is not extra light, but it is in category smaller libraries. One use case, that I would to support, is using in smaller application on smaller devices. For this purpose this library supports scrollable menu now. When the content of menu is longer then display, then the content will be scrollable - so cursor will be visible every time.