utils: gtk4: pop the secret flags menu at the correct place

Well, it's at least the same place as in Gtk3.
6 jobs for lr/popover-gtk4 in 21 minutes and 29 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Build
passed fedora28_dist #472107

00:08:06

 
  Test
passed centos7 #472112

00:13:22

passed fedora_autotools_full #472111

00:12:35

passed fedora_autotools_minimal #472110

00:02:16

passed fedora_meson_full #472109

00:12:05

passed fedora_meson_minimal #472108

00:02:08