Also consider a notification as unread only when it isn't active anymore.

notify-send foo bar, waited for popup to close, got a "1" in my panel
notify-send foo bar, closed the popup, didn't get anything in my panel
As suggested by VDG

I originally had the notification plasmoid show up alongside the popup to kinda indicate "this is where this thing is coming from/going to". With this change the notification plasmoid will not be shown in the panel unless there is an unread notification.