Improvement of the Fl_Sys_Menu_Bar class in relation to STR #3047:
- menu shortcuts, including function keys, are correctly handled
- the mac system menus and FLTK menus share Fl_Menu_Item's, thus 
many member functions of the parent Fl_Menu_ class apply equally
to an Fl_Sys_Menu_Bar.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10099 ea41ed52-d2ee-0310-a9c1-e6b18d33e121