app menu: standardize Help/About/Quit

* Remove the separator beneath About
* Rename Close Window to Quit

The action app.close is unchanged for the time being, since there's not
(yet) agreement on whether Quit will close every window or just the
current one. But it is going to be named Quit.

https://developer.gnome.org/hig/stable/application-menus.html.en

https://bugzilla.gnome.org/show_bug.cgi?id=720145