Show scrollbar like position indicator in mode line.
See the global minor mode `sml-modeline-mode' for more information.

Idea and part of this code is adapted from David Engster's and Drew
Adam's code in these mail messages:

  http://lists.gnu.org/archive/html/emacs-devel/2010-03/msg00523.html
  http://permalink.gmane.org/gmane.emacs.devel/122038

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