Fix desktop notification duration formatting

[DingDongSoLong4]

This is a simple fix for a bug with the duration formatting in desktop notifications.

As an example, a time of 2 hours 45 minutes and 12 seconds was being formatted as 3:165:9912 rather than 2:45:12, which I believe was the original intention.

This happens because Hours(), Minutes() and Seconds() give the total duration in that unit, rather than that specific "part" of the duration. The %02.f format string specifier adds rounding as well, which makes even the hours unit incorrect half of the time.

I've just fixed this by using time.Duration's String() function, with some conditional rounding to prevent displaying tons of decimal places. This does change the output format, i.e. it's now 2h45m12s rather than 2:45:12, but I think the new format is clearer anyway, especially with times shorter than 1 second (it now shows 425ms), and it means we don't have to manually do any math ourselves.