Fix problem with Fl_Tree::select_only() invoking select callbacks
before all other items have first been deselected.

Also: added some related demo code to tree-simple to demonstrate
'reselect' callbacks, where a callback can be invoked when an item
that's selected already is 're-selected'.



git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10015 ea41ed52-d2ee-0310-a9c1-e6b18d33e121
Fl_Tree:
	o Added new method Fl_Tree::get_selected_items()
	o Modified Fl_Tree_Item_Array to usable in a general way (i.e. beyond Fl_Tree's internal use)



git-svn-id: http://seriss.com/public/fltk/fltk/branches/branch-1.3@10016 ea41ed52-d2ee-0310-a9c1-e6b18d33e121