Retire app menu

As per Initiatives#4,
the application menu is being retired.

The application menu has been moved into a primary menu within
Usage, with "About" being changed to "About Usage".

Related to #52
1 job for wip/christopherdavis/remove-appmenu in 2 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #139497
build-gnome-usage

00:02:28