This way the user knows by the progressbar __and__ by the changing
number when the screenshot is going to be taken.
One second is added so the remaining time is never displayed as "0 s".
This led to the timer showing one second more than configured for a
very brief moment as soon as "Take a New Screenshot" was pressed.
To counteract this I substracted 0.1 seconds which isn't really
misleading because there is an additional delay after the timer is
finished (so Spectacle can vanish from view for the screenshot) anyway.
I hope this isn't too much of a hack.
Depends on D21638