[applets/devicenotifier] Re-add "show popup when new device is plugged in" option

This was removed in
!185,
however the original reasons for making the change no longer apply
because we now have a clean place to put the option--in the new popup
menu in the header. So let's re-add it, because hidden options are bad.
:)

BUG: 430017
FIXED-IN: 5.20