Update menus for GNOME 3.32 recommendations
See the wiki page for design guidelines.
- Remove app menu
- Rename About to About Documents and About Books
- Remove the Quit menu item
This looks a bit more complicated than some of the other GNOME apps.
In the reader mode, there is already a "primary menu". I guess we should try adding the "app menu" contents there unless someone has a better design idea. We'll also need to show that minimal "primary menu" in the overview mode.