Emit a signal when a user’s parental controls change

In the original Endless implementation, we could guarantee that a child account’s parental controls policy wouldn’t change while they’re logged in, because Endless OS doesn’t support fast user switching. So an admin would have to log out of the child’s account to log themselves in and change the policy.

That’s not true in general.

libmalcontent should be able to emit a signal when the current user’s policy changes, so that applications can reload it. This would essentially be a proxy of the org.freedesktop.DBus.Properties.PropertiesChanged signal from accounts-service.