Category selection in widget 'RECENT ARTICLES'

Add an entry to menu principale (the e is a mistype, not by me) for example.
Add a submenu for each category to be separately displayed.
I am not the inventor of that method.

To combine categories use plugin LastArticles where this particular feature is undocumented.

I accidentally found out that this plugin has the required capability because it uses the PHP function get_posts which allows “category” to be a comma separated order-independent list of valid categories. I tried and it worked right away.