Newsticker: Show feedicons in treeview. Small fix in opml export.

Newsticker: Show feedicons in treeview.  Small fix in opml export.

* etc/images/newsticker/README: Add rss-feed.png, rss-feed.svg.

* etc/images/newsticker/rss-feed.png: New.

* etc/images/newsticker/rss-feed.svg: New.

* lisp/net/newst-backend.el: Require url-parse.
(newsticker--get-news-by-wget): Store feed name as process property.
(newsticker--sentinel): Read feed name from process property.
(newsticker--sentinel-work): Rename argument name to feed-name.
Rename variable imageurl to image-url.  Pick icon url from Atom
1.0 data.  Launch download of feed icon.
(newsticker--get-icon-url-atom-1.0): New.
(newsticker--unxml)
(newsticker--unxml-node)
(newsticker--unxml-attribute): Documentation.
(newsticker--icons-dir): New.
(newsticker--image-get): New arguments FILENAME and DIRECTORY.
Use `url-retrieve' if `newsticker-retrieval-method' is 'intern.
(newsticker--image-download-by-wget): New.  Use process properties
for storing informations.
(newsticker--image-sentinel): Read informations from process properties.
(newsticker--image-save)
(newsticker--image-remove)
(newsticker--image-download-by-url)
(newsticker--image-download-by-url-callback): New.
(newsticker-opml-export): Handle url list entries containing a
function instead of an url string.

* lisp/net/newst-reader.el (newsticker-html-renderer): Whitespace.
(newsticker--print-extra-elements)
(newsticker--do-print-extra-element): Documentation
(newsticker--image-read): Optionally limit image height.  Use
imagemagick if possible.
(newsticker--icon-read): New.

* lisp/net/newst-treeview.el (newsticker--treeview-item-show): Limit height of feed logo.
(newsticker--treeview-tree-expand): Use feed icons in treeview.
(newsticker--tree-widget-icon-create): New.  Set the tree widget icon.
(newsticker--tree-widget-leaf-icon): Use feed icon.