Add stopwatch functionality

[dziemborowicz]

This would have to include common stopwatch features, such as a lap timer.

[redactedscribe]

Counting down is great, but sometimes timing how long things have taken is needed too. To time a task, one has to enable the advanced "Show time elapsed..." option and then set a time that is large, say 12 hours, to make sure the timer will not run out before a given task is completed. Alternatively, one has to use a stopwatch webapp.

Ideally a more convenient stopwatch method could be implemented into Hourglass.