When timer ends it should show the next start time (not stay at 00:00)

[dracan]

Now that we have different colours for 'pomodoro' vs 'break' mode (#59), it doesn't feel right the timer stopping at 00:00. Instead, it should reset ready for the next countdown. Note that this obviously only happens if auto-start break/next-pomdoro isn't enabled (#20). If these aren't enabled though, it shouldn't stop at 00:00, but should show that initial time of the next mode.