ui-utils: port the password storage helper to GtkPopover

GtkMenu and friends are all gone from Gtk4.
6 jobs for lr/gtk-4.0 in 10 minutes and 57 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed fedora28_dist #1577053

00:03:27

 
  Test
passed centos7 #1577058

00:07:29

passed fedora_autotools_full #1577057

00:03:12

passed fedora_autotools_minimal #1577056

00:03:22

passed fedora_meson_full #1577055

00:03:28

passed fedora_meson_minimal #1577054

00:03:15