use sub-second progress bar step size

[angli232]

The current step size of the progress bar is one second. However, it may not be good enough in the case of short videos with a total duration of only a few seconds. In that case, the indicator jumps too much each time and it is not possible to slide the progress bar to a non-integer time point.

I am working on a fix and will send a pull request after some more testing.

[lhc70000]

Thanks a lot!

I'm afraid that currently VideoTime.swift only works for integer seconds. So there may be a lot of works to do. Please update here if you have any problem.

[angli232]

Luckily time-pos could be get as double with the higher resolution, so the fix was not complicated. I am not familiar with Swift, so I hope I did not break other things with the changes.

[angli232]

Fixed by 63bf466.