org-download.el: more options for `org-download-delete'

* org-download.el (org-download--at-comment-p): new function.
  (org-download-timestamp): modified doc.
  (org-download--delete): new function.

`org-download-delete':
* called from the #+DOWNLOADED comment -> delete comment + link + file
* called with region active -> delete region + all inline image link files
* otherwise -> delete link + file for current line