Independent step forward button/step backward button times

[rathje40]

Both the "step forward" button and the "step backward" button use the "short-jump-size". DVR's usually have a longer step forward than backward, since you usually go back to hear something you missed. Ideally, these values would be independent and not tied to the hotkey values. An reasonable solution would be to be able to pick which hotkey value to use for each button. An acceptable quick fix would be to have the step backward use the "extrashort-jump-size".