Jump in video by a few seconds

Especially when observing behaviour in the video, it can be helpful to jump around in the video; i.e. jump back repeadetly to manually "loop" a certain section of the video.

This should probably be accessible via a shortcut. Current favourites are j and l, like in YouTube.

A future issue could be to make the interval user-managed. For now it will be a fixed time interval. @m.boltes will ask about the exact interval we will implement.