ui: Add help to shortcuts dialog

GNOME/polari!144
5 jobs for !144 with helpoverlay in 10 minutes and 33 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Commit Check
passed check_commit_log #588861

00:00:27

 
  Source Check
passed eslint #588862

00:00:41

 
  Build
passed flatpak #588863

00:09:01

 
  Review
passed review #588864

00:00:22

passed stop_review #588865

00:00:21