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