GNOME Shell 3.32 will remove support for the app menu
so we need to move its contents to the primary (hamburger)
menu.
widget-factory already had a primary menu.
The only item in the app menu was About.
https://gitlab.gnome.org/GNOME/Initiatives/issues/4
This commit add some more keyboard shortcuts to gtk3-widget-factory,
and adds a help overlay documenting them. This examle uses the
automatic resource loading support in GtkApplication.