Sometimes self.seconds is not initialized

[Jan Girlich]

After starting the program it sometimes refuses to work and displays these error messages on the terminal:

Traceback (most recent call last):
  File "./pomodorotimer", line 206, in start
    self.countdown()
  File "./pomodorotimer", line 209, in countdown
    if self.seconds>= 0:
AttributeError: PomodorotimerWindow instance has no attribute 'seconds'

I was not able to determine the source of this error or how to produce it reliably, yet, but I'm certain I can figure this out. Fix is going to be committed to my branch.