feat: show estimated remaining video duration

[WofWca]

We already have the "average playback rate":

And it should not be hard to estimate the remaining time by simply dividing video.duration by that playback rate.

The question is where to show this. The easiest way is to show this in the tooltip of the time saved element, as we do already with percentages. Another way is to add an option to show remaining time instead of the average playback rate, or not instead of it but besides.