Clean up wp_widget_rss_output():

* Fix appending [&hellip;] to $summary.
* Use wp_trim_words() instead of wp_html_excerpt().
* Trim $title before checking if it's empty. props UmeshSingla.
* Use correct escaping function for $title.

fixes #28356.
Built from https://develop.svn.wordpress.org/trunk@28586


git-svn-id: http://core.svn.wordpress.org/trunk@28411 1a063a9b-81f0-0310-95a4-ce76da25c4cd