Take file's time steps when "interpolate" option is uncheked and improve timeline scrolling

1) When the timeline slider scrolling steps are exactly the same as when playing video (previously scrolling step was always 1 hour whatever was the slices choice)
2) when the "interpolate between gribs" option is unchecked , the "slices per update" and "slices per hour" options are hidden (for that the setting dialog is slightly changed) and the file's steps are taken for both scrolling and playing video.
3) the "best forecast for now" is improved