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

This was removed in !185 (merged), 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

Edited by Nate Graham

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