o Added Fl_Tree::item_draw_callback(), letting one define a custom draw function for Fl_Tree_Item's.
o Added examples/tree-custom-draw-items.cxx to demonstrate its use.
o Fixed small doc error for recent Fl_Tree::get_selected_items()



git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10018 ea41ed52-d2ee-0310-a9c1-e6b18d33e121