def get_open_folders()
      expanded = []
      map_expanded_rows {|view, path| expanded << model.get_iter(path)[id(:path)] }
      return expanded
    end
def get_open_folders()
      expanded = []
      map_expanded_rows {|view, path| expanded << model.get_iter(path)[id(:path)] }
      return expanded
    end
returns an array of open folders.  The array of folders can be saved, and then you can
 pass the array to #open_folders to restore the state of the file tree.
 @return [Array] Returns array of strings with the full expanded paths of the open folders.