Simplify the "duration" timecode to be human-readable

[nekohayo]

Accurate timecodes make sense for in/out points, but not really for duration, which is too hard to parse quickly.

I posit that nobody does individual subtitles that are more than 60 seconds long (or even 10 seconds, most of the time). Therefore showing this in timecode form (such as "0:00:03.953") in the "Duration" column (and on the tooltip when dragging the in/out point) makes no sense.

It would be much easier to parse (and more elegant) if it was showing "3.953 secs" (or "3.953 seconds" if you want to be exhaustive) instead. If you're worried about 9 vs 10 seconds, right-align the column's text in the gtk listview. In the case of the waveform interactive tooltip, you could prefix that with "Duration:\n"