bug#24073: 24.5; outline-on-heading-p sees any invisible text property as outline inviisble

[Paul Rankin]

A better solution is to make `outline-invisible-p' return t only if invisible 
text property is `outline'

    (defsubst outline-invisible-p (&optional pos)
      "Non-nil if the character after point is invisible."
      (eq (get-char-property (or pos (point)) 'invisible)
          'outline))