Added some missing Doxygen comments in the Fl_Sys_Menu_Bar class.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10101 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Completed the implementation of Fl_Sys_Menu_Bar that now accepts all calls of its parent class Fl_Menu_.

git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10102 ea41ed52-d2ee-0310-a9c1-e6b18d33e121