Added _next_sibling and _prev_sibling to Fl_Tree_Item class to make
next_sibling() and prev_sibling() more efficient during item selection.
Used new FLTK_ABI_VERSION macro (as designed by Greg and Albrecht on fltk.dev) to protect the ABI breaking features.
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@9231 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
o Added user_data() to Fl_Tree_Item
o Added insert() and add() methods that allow specification of Fl_Tree_Prefs
o Changed Fl_Pixmap args to Fl_Image for more flexibility
o Fixes for positioning of items in the presence of user icons
o find_children() changed from protected -> public
git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@6956 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
