Move global menu (appmenu) back to menubar

AppMenu was really used by Gnome Shell only. Since its retirement with
Gnome Shell 3.32 there is no need to keep the 'prefers_app_menu' switch.
3 jobs for join_menubar in 2 minutes and 43 seconds (queued for 4 minutes and 25 seconds)
latest
Status Job ID Name Coverage
  Test
passed #14595
run-appdata

00:00:06

passed #14594
run-tests

00:02:28

 
  Build
passed #14596
run-build

00:00:08