Extending the work / rest time, during a cycle

[lcb931023]

If the user does this during the first cycle, it would work as expected: the time gets extended.

However if it's a few cycles in, the extension would be wacky because it's extending all of the previous cycles' time.

We should change this to only extend / reduce the time in this current cycle.

This would mean having to change the start time too, ruining the sync between shared timers. But maybe that's alright because adjusting work / rest duration would ruin the sync anyway.