Preserve group structure on opml import and export.

Preserve group structure on opml import and export.

* lisp/net/newst-backend.el (newsticker--raw-url-list-defaults),
  (newsticker-url-list-defaults),
  (newsticker--get-news-by-url),
  (newsticker--sentinel-work),
  (newsticker--parse-atom-0.3),
  (newsticker--decode-rfc822-date),
  (newsticker--image-download-by-wget),
  (newsticker--image-save),
  (newsticker--image-download-by-url),
  (newsticker--cache-save),
  (newsticker--stat-num-items): Fix indentation.
  (newsticker-opml-export): Preserve group structure on export.
  (newsticker--opml-insert-elt),
  (newsticker--opml-insert-group),
  (newsticker--opml-insert-feed): New.
  (newsticker--opml-import-outlines):
  (newsticker-opml-import): Preserve group structure on import. (Fixes
  fourth issue in Bug#41376.)