Seconds roll over detection could fail in Kiwi timestamps in PAL videos

[fcasarramona]

In the function def format_kiwi_timestamp(ts_str, t2fromleft): to detect if a second roll over occurs the following code is used:

        seconds_rolled_over = (ff_left < 17 and ff_right > 300) or (ff_right < 17 and ff_left > 300)
        if seconds_rolled_over:
            use_ff_left  = (ff_left  < 17 and ff_right > 300)
            # use_ff_right = (ff_right < 17 and ff_left  > 300)
        else:
            use_ff_left  = ff_left  > ff_right
            # use_ff_right = ff_right > ff_left

This is correct for NTSC videos, because every field lasts 16 miliseconds, but not for PAL videos, where every field lasts 20 miliseconds, so the '17' should be set to '21' at least.