This replicates the old interface from Endless’ gnome-control-center,
with no attempt to improve or rework it. That will come later.
It might not work fully.
It allows a user to be selected, and their parental controls to be
changed. It currently doesn’t filter the users.
It supports a simple ‘Loading’ screen, before displaying the main
interface. If loading fails (due to a D-Bus error with accountsservice),
an error page is displayed instead.
Signed-off-by: Philip Withnall <withnall@endlessm.com>