collection-header-bar: Update primary menu layout

Remove separator, change 'Shortcuts' to 'Keyboard Shortcuts' and
'About' to 'About Games' to conform with
https://gitlab.gnome.org/GNOME/Initiatives/wikis/App-Menu-Retirement
6 jobs for wip/exalm/appmenu in 21 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #118090
flatpak:app

00:12:26

 
  Test
passed #118091
flatpak:tests

00:05:37

 
  Review
passed #118092
review

00:00:29

passed #118093
manual
stop_review

00:00:31

 
  Cross Distro
passed #118094
fedora:29

00:02:50

passed #118095
ubuntu:devel

00:02:16