org-indent: Fix indentation with proportional font

org-indent: Fix indentation with proportional font

* lisp/org-indent.el (org-indent-max):
(org-indent-max-levels):
(org-indent-strings):
(org-indent-stars): Remove unused variables.

(org-indent-initialize): Remove function.

(org-indent-boundary-char): Remove unnecessary comment.  Do not rely on
function above.

(org-indent): Make sure characters used for virtual indentation are
invisible since they are not necessarily white spaces.

(org-indent-set-line-properties): Fix indentation with proportional
font, i.e., do not use only white spaces to indent.
(org-indent-add-properties): Apply changes above.

Reported-by: Jakub Szypulka <jakub@szypulka.de>
<http://permalink.gmane.org/gmane.emacs.orgmode/100252>