Improve compatibility between org-indent-mode and visual-line-mode

Improve compatibility between org-indent-mode and visual-line-mode

* lisp/org-indent.el (org-indent-mode): completely refresh buffer
  before starting org-indent-mode. Also set idle timer to refresh only
  visible portion of buffer, and refresh the subtree instead of
  section when promoting or demoting it.
  (org-indent-add-properties): rewrite function to proceed line by
  line, as required by `wrap-prefix' specificity.
  (org-indent-refresh-section,org-indent-refresh-subtree): refactor.
  (org-indent-refresh-view): new function.
  (org-indent-refresh-to, org-indent-refresh-section): removed
  functions.
* lisp/org.el (org-unfontify-region): do not remove prefix properties
  when unfontifying a region.