menus: standardize help/about/quit

Help, About, and Quit belong at the bottom of the app menu, in their own
section.

https://bugzilla.gnome.org/show_bug.cgi?id=742063
menus: don't duplicate menu items

Items in the app menu should not additionally be available through the
window menu.

https://bugzilla.gnome.org/show_bug.cgi?id=742064