- Fix gimp_scroll_adjustment_values() for smooth scroll events
- Set GDK_SMOOTH_SCROLL_MASK on all widgets where we set GDK_SCROLL_MASK
- Add GIMP_ZOOM_SMOOTH to enum GimpZoomType
- Add "gdouble delta" to gimp_zoom_model_step()
- Change the meaning of the "scale" parameter to "scale or delta" in
  all functions that take GimpZoomType and a scale factor.