do_callback only if callback exists


git-svn-id: https://geuz.org/svn/gmsh/trunk@16717 59c8cabf-10d6-4a28-b43f-3936b85956c4
remove submenu buttons until fltk bug is fixed


git-svn-id: https://geuz.org/svn/gmsh/trunk@16718 59c8cabf-10d6-4a28-b43f-3936b85956c4